I'm using a SimAudio 740P preamp with my 5.5, and they make a great pair! My favorite amp up to this point with the 740 was the Pass X250.8, and I think this pairing is just as good overall if not better. I don't personally see it as a hard requirement for a lush or tube preamp to pair with the 5.5, not that a resolving tube preamp wouldn't also be a great match. I don't find the 5.5 forward or fatiguing at all, and that is after owning the warmish XA30.8 and the very easy on the ears AtmaSphere Class D monos. Personally, I think it would be a disservice to add a preamp that masks resolution and detail with the 5.5 as its so clean and resolving with an incredibly refined high end. Why hide it? :)
