What's next for the ultimate sound quality?

You describe two shortfalls: soundstage and imaging and thin sound.

The soundstage and imaging could be improved with the acoustical treatment as suggested by Mdowns32. You are probably getting alot of sound reflection which blurs the image and soundstage. Also, make sure that the speakers are equi-distant from side and back walls. Also, play with toe-in angle.

Thin sound: You might try putting the speakers closer to the corners or at least closer to the back wall.

Other than that, you are into trying a number of things including different amplification - I have always preferred tube and there are a number of different designs using different driver tubes that can affect the sound. I have gotten the impression from reviews on digital switching amplification is that it can sound thin. You might want to try different cables. Good power cables can make a huge difference. The DAC recommendation is also very valid. Finally, controlling vibration and resonance is also important.

My suggestion would be try to locate the closest audio society in your area, listen to different set-ups, and possibly borrow equipment from other members to see what the effect is of different equipment, cables, etc.

You will get a lot of good recommendations from the Audiogon forum but it comes down to gaining experience with different types of equipment - solid state, tube, and digital switching amplification; effects of cables, effects of good transports, effects of vibration control, and so on...

Good luck and enjoy the journey!

If you go with a kit for amplification, check out the Audio Note kits.

I bought a used Sugden amp here and it makes an astounding difference in the sound of my speakers. Extremely warm, natural, beautiful. The sound from my Denon receiver is nice for video or background music, but never for critical listening. It sounds thin and forced. A good amp will let the music just flow, and when you close your eyes the speakers will disappear.

What's next? Electrodes into our cortex sending sound in directly.

I'm reading this post quite a bit late, but I wanted to put in my two cents and ask questions, as I've been really contemplating building the 2-way version of this kit.

IMHO, the source will make the biggest noticeable difference, followed by amplification. That said, if it were me, I'd upgrade the amp first. Due to the efficiency and easy load of these speakers, you'd be doing yourself a huge disservice by not trying out some single ended or low powered tubes, as in something in the 10-30 watt range. You will be very surprised how powerful those little watts can sound on decently matched speakers. They can be had for fairly cheap on the A'gon. I found tubes to really shine in terms of 'presence'; not as pinpoint in the imaging, but much more 'real' sounding. If you want a 'tubey' sound in a solid state, a safe bet is Pass Labs. They have great resale, and sound tube-like for solid state. I'll do a little Krell bashing here. They just don't sound good to me. Very thin, analytical, but powerful. Same goes for Levinson.

It's too bad you haven't heard more gear. I'm considering getting Merlin vsm, because they suit my needs at the moment. At one point I swore I'd never get them, but I'm really curious as to how they will sound. But I really wonder how they would compare to the ZRT's, a good design speaker with top of the line components. Anyone out there do anything remotely comparative? A friend of mine has some diy monitors using Dynaudio drivers and really likes that sound. I just wonder how much "The whole is more than the sum of it's parts" applies to something like the Merlin vs. the ZRT.