The FP2600 does around 600Watts @ 8 ohms (I'm guessing based on 840W@ 4 ohms), pretty close to the powerhead. C Kemper has stated the powerhead amp's 600w is equivalent to a 100W valve head. Plus Lab Gruppen has a pretty good name for clarity.
Also, it's the 2nd cheapest. The Yamaha one is a bit underpowered I think. Go the larger Lab.Guppen if money doesn't matter, the tone should be the same, just an amazing amount of volume (careful with your cab!)
Pity there's no Matrix suppliers in Oz to test, that would be probably have the most valve-like reponse for a SS amp.
I don't think it's possible with the CLR, it has dual inputs only I believe, no ouputs IIRC? How are you finding the CLR.?, I've been considering getting a stereo pair for live and studio use.
I'd go monitor out (cab sim off) - mesa fx return in - 4x12
and the balanced out going to the CLR.
edit: checked the manual, the CLR link output is only a clone of the incoming signal, not powered of course, so sorry, it won't power your 4 x 12.
I use the behringer EPQ900 through a 2x12 at 8ohm. Plenty loud enough. Used it through someones 4ohm cab the other night and it was ridiculously loud.
I've had one for about 2 weeks now, haven't rehearsed or gig'd yet, still using my studio monitors for reference when monitoring. I've only used it after I've profiled and so far I'm really really impressed. I wanted to be able to simply be inspired when I play through it and so far it's been so great in that regard. I've mainly used it in the tilt position, haven't really experimented too much with the other positions. One of the best things about it is that it sounds pretty the same when I'm moving around the room. I'd love to be running two of these in stereo, that would be fantastic.
I'd like to really venture in both the SS and tube power amps running through cabs or a passive CLR. That Fryette LXII looks great, so does the Atomic amps SLR400. Never tried a tube power amp, that's why I'm keen to set up the guitar amp Kemper combo to at least get a feel. I'm mainly hearing it's an in the room feel that's the difference, not so much the sound/tone. Mainly wanted the Kemper and CLR combo to make it easier for gigs etc, but have been considering selling my amps, that's how impressed I am thus far. Keen to compare....
You should bare in mind that tube amps aren't good for pairing with the KPA, unless you specifically use profiles which haven't included the power amp section. By default any KPA profile includes the power amp section, so if you use a tube power amp with it, you're getting 2 power amps affecting your tone, not ideal.
Well, as long as you don't push the power amp too much you'd not make a bigger mistake (in terms of linearity) than using a guitar cab 
I've been getting great results by shutting off the cab block and locking it, then just going through and selecting a new set of favorites that all use my Marshall with V30s.
These are powered by my Mesa stereo 50/50 power amp.
It has a switch to 15 W per channel, so I can try the presets with heavy tube coloration or with less on the 50 per channel setting.
This is excellent for tube-driven metal sounds. I just love the sound of my Marshall, as I have for years, and I can run all kinds of different amps through it now. When the time comes, I'm sure I'll get a FRFR/solid state set up to try, but the balls and the tone coming out of this setup from a lot of the profiles is awesome.
Of course, these aren't the tones intended by the authors, but who cares? The results are killer. From Mesa Boogie to ENGL to Plexi amps it's just awesome. Don't let anybody tell you it won't sound good. That's not the way this beast works. If you like the way those heads sound through whatever cabinet you've ever heard, you'll equally enjoy the results with the Kemper, the way the 50/50 pushes it with 6L6's.
Yeah, that's the beauty of the profiler.
