Opinions Please - KPA using EVH5150III 50W as Power Amp (Effects Loop Return) and EVH2x12 Cab

Welcome.
It's definitely not predictable if your results with your setup will be completely satisfactory.
But as you want to play metal profiles through a metal setup I'd speculate the results will be quite good.

You have to try yourself.

In the end only a neutral sounding FRFR setup allows predictable results.

I used to have the EVH 50W head that I used to use as a power amp for my Kemper. I thought it sounded great, especially with Marshall style profiles.

I actually sold it to fund a Triple Rectifier coincidentally. In my opinion you'd struggle to get a decent Rectifier tone with the EVH head, as a lot of the tone comes from the power section of the Mesa. You could possibly get close preamp wise but you'd struggle to get that "wide open" Rectifier sound.

Now, if you're recording / going FRFR it's a different matter of course!

Hi, welcome
Do you like Aprilia cars?

Just use your common sense here: the results using an external guitar amp will be less faithful as long as the amp\cab you use sounds different from the rig you want to emulate. This doesn't imply you'll sound unpleasantly, as long as you like what you hear.
The sound you'll get from the setup you've described will certainly be good per se. So my advise would be to try a Profiler and see if you like the results with what you own now: if you get 3-4 usable sounds you're good to go (it will be more versatile that a single amp anyway). Then go to an audio store and try a good PA with your Profiler and see whether it makes sense to you to sell the amp to fund a linear cab.
I'm pretty sure you'll be anyway fully satisfied with one of the two stages
When the worse comes to the worse you can return the Profiler and call it a day

I remember Michael Britt's YouTube talking about his setup. Looks like he is using his 5150 just to power his onstage 2-12. He has the KPA output knob controlling just his onstage volume.

http://www.mbrittprofiles.com/my-rig.html

If you're wanting it to sound "right" coming out of the speakers I don't know that I'd recommend it because there is some coloration from the extra tube amp and cabinet. I run direct out of the Kemper to the p.a. and that's what I hear in my ear monitors so I don't "need" it to sound right coming out of my stage amp. I just need to feel some punch and some air moving. If you're only using it that way, you can probably eq the monitor out and the profiles to get close. The benefit of using a tube power amp with the Kemper is that it feels more like a real tube amp because of the punch from the tube amp as compare to solid state amps. The trade-off is that the profiles don't sound exactly the same coming out of a tube amp and guitar cab. It's really just a matter of experimentation and trial and error to see what works for you in your situation. I tried using my Atomic Reactor cab onstage and I missed the power of the real tube amp, esp in the low frequencies.

I have used a Mxr between the Kemper output and power amp with good results. You will not reach neutral but it will allow you to sculpt the sound.

Quote from aprilia4life

I have an MXR 10 band EQ, could I use that to EQ out some of the EVH poweramp/cab sound colouring and therefore get a more neutral sound using the amp/cab setup?

The correct answer would be "no". First of all you'd need measure instruments to determine what ranges of frequencies needs to be tweaked; then you should hope that your MXR's frequencies correspond to the ones you'd have to modify (forget about it) and the sliders have the same freq range (forget it twice).
You'll of course be able to drastically change the overall sound as deadpan pointed out, but certainly not towards a better linearity.

Quote from aprilia4life

My main interest is in high gain metal tones, I wanted to get a rectifier as my next amp, but thought one tube amp is enough, so going the way of the KPA. So cleans/rock tones are far less important to me. I am not *too* fussy when it comes to amp sounds, I just know what I like.

Welcome

I think you answered some of your questions here. Try frfr, try cabs, some people are using home stereos (perhaps not the best idea for high gain). There is really no right or wrong answer, and if you have a good idea of what you are after, the method to get there is secondary.

"Do you like Aprilia cars? "

This reminds me of the old story about 2 people talking about which car is faster, handles better, etc. with a racetrack right in front of them.... easy way to find out. Long story short, especially if you are on a budget, do what Viabcroce said, go check out some PA's after you found some proffiles you really like (headphones, amp/cab, whatever). While at said music store also play your KPA thru any amp that catches your eye. If it is a good store, let them know that you are testing how it sounds thru all sorts of things, and are getting your bearings. Some there might even have some good suggestions once they hear what kind of sounds you are into. EQ to taste. A good salesperson in any field will try and find out what you want, and use their experience to make you happy. Spend a day in a decent shop and get ideas.

There is also new firmware in the workd that does better cab separation, DI profiles, making it even better to play thru a cab, lots more. Get thru basics first, then once familiar, keep exploring.

There are a lot of possibilities with this device, take your time, and enjoy looking into as many of hem as you see fit. Don't worry about correct so much, if your ears are good, rely on them more

Quote from aprilia4life

First things first, Aprilia's are motorbikes

Yes. Legendary.

Quote from aprilia4life

I am concerned that if the EVH PAmp/Cab setup doesn't sound too good, I am left with a KPA without the built in power amp.
What are my options after that? Would I have to buy a power amp with a neutral sound and monitors? Through my stereo? Through my Yamaha THR10X?

Well, there are so many options. You could buy neutral sounding monitors which is by far the most recommended method of amplifying the Profiler. Or you could buy an external poweramp and another cab. Or you could retrofit your unpowered profiler with a 3rd party poweramp.

Quote from aprilia4life

First things first, Aprilia's are motorbikes

Secondly, I am in Australia, so Kempers are pretty rare, return policies are non existent, and shop staff are generally INCREDIBLY useless.

I am concerned that if the EVH PAmp/Cab setup doesn't sound too good, I am left with a KPA without the built in power amp.

What are my options after that? Would I have to buy a power amp with a neutral sound and monitors? Through my stereo? Through my Yamaha THR10X?

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Not really
http://www.museoauto.it/websit…ia/111-lancia-mod-aprilia.
My father used to have one, and there's a huge fanbase still today
I know the motorbike brand as well, they're both Italian concepts.

If you order a Profiler from the Internet you'll have a return policy.
As for the amplifying possibilities, as Ingolf said you'll have plenty of them. So I'd buy the Profiler from a site (say Kemper themselves), spend some days with what you have now, and meanwhile visit a good audio store. You do not strictly need advices, just good stuff to try. Be sure to have learned the basics about the Profiler to test it at its best. For example, start by excluding the cab profile when using guitar cabs, and remember to set it to on when using a linear system.
It's almost impossible you won't find a tone setting you'll like IMO

I use my 6505+ combo as my power amp. Then I plug my kemper's monitor out into the fx return of my 6505+ and I turn off the cab sim on my Kemper and it sounds brutal. I have a v30 in my 6505+ and it's bad a**. The kemper is a great amp, keep rockin on man! \m/

I used my Kemper live for a year with a FRFR QSC powered cab and never could get used to it.. I'm a rock n blues artist.. I
last tried a power amp and guitar cab... Much more to what I like to hear... Gotta try it.. Only u will know