With the choices you have narrowed down.. They're all going to be quite good. At that point, it becomes fine tuning to your actual needs. Your speakers and room setup and other components will come into play with that. One amp may come across as "warm", and the other maybe a little more "lean" or "bright". You'll need to listen to them to come to that conclusion.. So if you consider your speakers a bit more "warm" than neutral, you wouldn't want to match up a "warmer" amp with them. etc.
Were it me, of the choices you have, I'd be happy with any of them, unheard, based on other reviews of them. The Theta is supposed to be a beast! as well. If you're bumping into that league, consider also Balanced Audio Technology (BAT). The VK 6200 also appears to be a giant killer. I have a friend who picked up an integra research amp, for one reason because BAT was involved with the design. Claims it "lifts a veil" compared to his B&K 7270 II which is not a bad amp at all.
To answer your question though, narrowing it down, I'd listen to them. They should ALL be pretty much "straight wire with gain" meaning they should get out of the way and simply amplify, without adding color.. Especially in this league.
If I were to rank your choices, it would be:
P5 / A51 (both tied a close 2nd)
Also, one thing to keep in mind.. If you're considering a pre-pro later to match up, the Anthem Statement unit is getting some good press. Though I wouldn't have any issues looking at an all Theta rig in my rack, at all!