I agree with inna. Even though you think that your amp is neutral (I like the word natural better), there is apparently a subtractive coloration that is making the music less colorful and interesting, even though you can't identify it.
For some years, I used a McIntosh MC 300 as my main amp. I really liked it, and I had retired my MC2105 in favor of it. One day I took out the 2105 just for fun, and I never went back. It wasn't as silent or modern, and the mids were a bit more forward, but even though I'm sure that the MC300 was technically superior, I enjoy the 2105 presentation more. To sum it up, I think that every amp has a personality, and I don't think that the Sim is the right amp for you.