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 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.