Pass vs Simaudio

Not familiar with the X250.5, but very familiar with the original X350. Pass has the edge in warmth, if that's what you like. The Moon has the advantage in deep bass reproduction, with more slam and control, especially with difficult speakers. Overall, it's pretty close, but I'd take the Moon for its overall bigger soundstage and, I guess, somewhat more accurate reproduction. I depends a lot on your speakers too, though. Both are great choices.

I guess I should have added a McCormack dna-500 to the mix

Wouldn't be a bad idea to add Ayre as well.

I don't know much about the DNA-500, other than its good reputation. I wouldn't, however, include an Ayre in the mix. While, IMHO, good sounding amps, they seem to have difficulty with complex speaker loads and can sound clean and extended, but somewhat constrained. I am told that they use smaller power supplies, which limits their applications. With easy-to-drive speakers, they sound great though. I think it will be very difficult to surpass the huge power supplies used in Moon and Pass amps. Although an older design, check out the Rotel RH-B10 "Michi Series", if you can find one. No relation quality-wise to what they build today. They're extremely rare, sell fast, and sound amazing, though not quite up to the league of the Moon/Pass. However, I would put the B10 ahead of the Ayre. Again, it's all relative to what kind of sound you want.

I have a DNA-500 (currently retired) that I used briefly with my VSA VR-7's (and before that with Legacy WHispers). Great solid state amp, lots of power and musicality. It easily drove the 7's with no problem. Replaced with a VAC 300.1 due to great synergy w/VSA and desire to try the best tubed amp I could find for my application. I am not selling the DNA-500 - its is that good at its price-point.

If you haven't heard the new Ayre gear, your comments are irrelevant.

Thanks guys. I have looked on multiple boards and can't find one person who didn't have really good things to say about the dna-500 and every comparison put it above the Pass and Simaudio. I think I will go with that. The other alternative would be to biamp with a tube amp for the top.

If your choice was DNA-500 i know you will be very happy and satisfied , had mine about 2+years with no looking back or a thought of a upgrade, that's always creeping in my mind!

I've had my Pass X250.5 for almost 2 years now. I'm extremely happy with it. I've always used Audio Research tube preamps with my solid state amps though.
Prior to the Pass, I had McCormack DNA-225 stock, then had it modded to almost platinum(everything done minus XLRs).
Pass is better. It is more effortless, dynamic and more true to life sounding. It seems like it has no limits even with my not so easy to drive B&Ws. I haven't heard Simaudio Moon next to Pass so can not offer any thoughts on how they compare. One thing I know, I have no desire to try any other amplifier. And that's for almost 2 years now which in this hobby is almost like eternity :)