I've had the AC-4 in my system (see previous post above) for just over a week now. Here's what's going on:
1. Right away I noticed a drop in the noise-floor: honest-to-god-dead-silence between tracks. This initial impression has been confirmed over this week by my being able to play, well, everything, louder with no down-side.
2. On disks (yes, I'm a CD-only guy) with "hot" recordings, there's a greater sense of "ease" with, say, sibilants, and instruments are "more themselves" in their tonal signature.
3. Soundstage is improved in the placement of instruments and vocals: more dimensionally right, more solid, and then also a little more forward. I'm not sure what's going on with this "more forward" so I'll pay attention to this over the next couple of weeks.
4. I agree with the comment on dynamics not being affected. This was my single greatest concern but Dusty seems to be quite right about the 10A socket: I prefer the Pass XA-25 with the AC-4 than "straight in."
Finally, I worked with John Casler to get the AC-4, and will be getting future NuPrime products with his help.