I agree that the Spectral may not be the best match because of the Spectral preamp issue. I haven't heard a Boulder amp in years so I'm really not sure if that would work well. I will say that I think they were a little overpriced for what they give you. I don't know how different their newer models sound, though. And that leaves the Pass. Its a great amp, but if you look at how its designed, its very different compared to your Mac preamp. So much so, that I think the 2 pieces would be working against each other. For example, Mac believes in using a lot of neg feedback, where Pass doesn't use any. Pass believes in using a very simple design (very few components in the signal path). McIntosh is just the opposite. Looking at it that way, its easy to see potential conflicts. I'll use one of my systems for another example. The components are made up of several different brands: Ayre, Wadia, Aesthetix and Vandersteen. Even though I'm using 4 different brands, they work well together because they are designed by companies that think in a similar way. If I took your Mac preamp and put it in my system, it wouldn't allow the other components to sound they're best. If I took the Pass amp and put that in my system, it would be a much better match. It wouldn't take the fully balanced, 0 feedback, in phase signal coming from my Wadia and Aesthetic front end and turn it into something else. It would retain those properties as it continued to the speakers. The Mac preamp would introduce feedback to the signal. Feedback also alters phase. So just putting 1 odd component in a system can have a serious impact on SQ.