The Matrix seems very appreciated, apparently sounds like a tube amp....
It depends on how much, you want to invest.
I use most of times
a VHT 2 90 2
and Randall RT 2 50
Sometimes I use a Peavy Classic 2x60, but IMO the best sound comes out of VHT. Huge tone!
With Randall I have versatility because of different pair of tubes in every channel (Ch 1 EL34 tubes, Ch 2 6L6
tubes) and channels are midi switchable
Lower budget PA I have used are, Marshall EL84 2x 20, and Mesa 2x20. IMO Mesa is better than Marshall.
Before you buy is good to try them if it's possible, and decide yourself what match your needs. 
If you really want tubes I'd get a VHT because they have very little coloration... others like Mesa will color the tone too much for this task ...
My choice would be the 2902 VHT
I am thinking of buying the Matrix.
you never used yours?
Thanks for the suggestions guys,
No I haven't plugged it in yet... was using it for my PA system. But I will give it a try at lunch to see if there's more punch/power.
CH
Cool, please let me know.
I am getting ready to order.
Definitely go for the Fryette 2-90-2, very cool and LOUD amp!
I tried that with my Matrix GT800FX and with a VHT 2502 and got NO decent results by only use the monitor out with switched off cab sims. It is always "some cab" in the signal which leads to a somewhat weired sound, compressed, no punch. The only thing that works for me with amp/cab are the DI profiles.
But after a lot of messing around with different setups I am now very happy with KPA -> RCF NX12SMA.
Agree, the monitor out with cabs switched off does not worked properly but with a the 'cab bypass' cab and cab sim on I have great sounds with the Matrix. My cabs are TT custom cabs loaded with EVM12l.
I use a a ENGL520 as well as the Power Amp of an ENGL Rackhead 860. They both use use EL84 tubes and sound great with the DI profiles. But without the proper function of the "cab off" this is not living up its possibilities.
Has anyone even directly mailed the kemper team about his? Perhaps we should have a little petition on the forum and then mail it to the kemper team.
I'd been thinking about this too, particularly after reading the thread where one person asked "how does Till profile his cabinets?"
Part of what we consider to be the "sound" of a guitar cabinet in most commercial recordings has to do with the EQ coloration inherent to tube power amps caused by differences in damping factor. As usual, the response of solid-state gear is more accurate and "correct" (i.e., closer to the manufacturer's lab test speaker response), yet the increased resonance we get from tube power amps can result in a more traditional tone. That brings up the question as to whether, if one is profiling strictly for cabs, should one should stick with using a flatter/tighter solid state power amp or the more traditional (flabbier/thumpier) tube power amp instead. It's probably too much to expect the KPA to be able to separate the anomalies of the power amp from the actual cabinet in this case; the manual is silent on this subject.
I'll even pass along a link of how one Axe-FX guy tries to compensate for this in his own patches - there's something for everyone here:
http://matrixguitaramplification.com/tweakistry/
I imagine the manufacturer's official response might be along the lines of "The profiling process is meant to replicate the entire signal chain, so you're on your own when strictly profiling cabinets" but it applies when using the KPA on stage with real cabinets and power amps too. Should the KPA have a "Tube Amp Resonance Correction" button when playing the KPA live through a solid state amp along the lines of a "loudness" button on your stereo? Redwirez addresses the problem by profiling their cabs as if driven by solid-state amps and allowing the user to apply a "tube amp correction factor" filter, which is adjustable.
But lacking this, I can only imagine - since tube power amps are still the most expected case for recording and old-school live situations, that if you're going to profile a cab for "universal" purposes you'd probably be best off by driving it with something like a (tube) Randall RT2/50 and maybe labelling this fact in the "Cabinet Comment" section of the cabinet profile Tag fields.
Thoughts 
-djh
The question whether to use tube or solid state amps for cab profiling is just a matter of taste that everybody should do to his own liking. The "cab off" problem is totally different thing, because it makes the sound not just a little different that might easily be corrected with the monitor eq. It leaves some severe tonal shaping in the signal path that just makes it sound "wrong" when compared to a DI profile. It is a little bit like having two speaker simulations on at the same time. I see two possible reasons for this.
1. It is really by intention like CK stated. Then we all should really make clear to make a "totall cab bypass" function available.
2. The Amp/Cab separation does not work as perfectly as it should. It this should be the case, one might rethink the idea of seperare preamp / poweramp with cab profiling, using the additional inputs and outputs on the kemper.
On a second thought, number 1 seems more likely. Because if you use DI profile and then add a cab, the "cab off" function on the monitor does not still work correctly, right? I am not sure at the moment. If this is the case it evident that it must be by intention.
I remember that too. There was a related thread where I wondered whether you'd get better results profiling your amps and cabs separately and combining them, vs. together. Would it be right to say that if you're ever thinking of swapping out your amp or cab in a rig then for best results the amp at least should be profiled as DI? Correspondingly, if you're never going to be doing this, then the results of profiling them as one might be best?
-djh
I remember that too. There was a related thread where I wondered whether you'd get better results profiling your amps and cabs separately and combining them, vs. together. Would it be right to say that if you're ever thinking of swapping out your amp or cab in a rig then for best results the amp at least should be profiled as DI? Correspondingly, if you're never going to be doing this, then the results of profiling them as one might be best?
-djh
Yeah, this would be exactly what i think. If you plan to swap the cabs anyway, then profile it as DI. The same if you plan to use monitor out with "cab off". If you use it for FRFR or just recording and you want your authentic tone then you should profile it in a single pass.
I use a a ENGL520 as well as the Power Amp of an ENGL Rackhead 860. They both use use EL84 tubes and sound great with the DI profiles. But without the proper function of the "cab off" this is not living up its possibilities.
Has anyone even directly mailed the kemper team about his? Perhaps we should have a little petition on the forum and then mail it to the kemper team.
I have reported this as an issue...
Yes, I reported it back in January.
