I should have mentioned I have a 5500 in another system driving some Dynaudio 60's. I think they are only 86dB efficient and this amp drives them to "very loud".and if your speakers can handle it the 5500 does big bass. The 555, is an earlier model and closer to the Pass original design if not, the original, I'd have to do some checking on the geneology.

Gerall, I'm just using a gtp-450 preamp. How's the Aric Audio sound? Does it have remote, which would be a must for this system as it's hard to reach from the treadmill. I've thought about doing something different with the preamp but the 450 isn't bad and has the tuner built in. I was thinking about going up to the 750 but I auditioned once and didn't hear a lot of difference between the 450 and 750. After seeing and hearing reviews of the 750 I wondered about the audition if something was wrong. But you know how something is said, and Stereophile gives it a nod and all the sudden it's good whether it is or not. I don't see many of them listed in systems and usually one or more are on Audiogon. The only real criticism I have of the 450, and you really can't criticize it because it is great for the price it was new and a steal used, the highs I'd like to be smoother, or less harsh, and a bit more resolution would be nice. The 5500 has an interesting sound, sort of a dark feel, I wonder if tubes would take it too far in that direction.