Hey Barry, it's reassuring to see that someone here is even crazier about this stuff than I am (though maybe I don't really need the encouragement...).
I'd like to follow up on several posts here, but need to get some work done this afternoon, so for now I'll just comment on the sloppy nut approach.
I'm sure loosening the nut screws will make a difference, but it's not something I would recommend, or bother to experiment with myself. Ideally, the nut only matters when you're playing an open string. Assuming you're fretting with reasonable pressure, there shouldn't be a lot going on up there the rest of the time (though of course the whole neck is always vibrating quite a bit, transferring some of that to other strings perhaps, etc.).
Also, whenever you talk about leaving something a little loose, it's hard to be repeatable, and likely to shift over time. So my preference is to experiement with materials, but keep everything nice and snug.
I did start playing with alternate blocks, but have only had a few hours so far; I'm not being very scientific or rigorous yet, just going for general impressions, and allowing myself to change other things if I feel like it, such as pickup heights and tone/filter controls.
No question, block material makes a big difference. Wood (only vermillion so far) was thumpy and sort of boring. Corian (counter top stuff) was quite interesting, very clean sounding at first reaction, though I came to feel it wasn't heavy enough to provide a good low B. Last night I put aluminum in there, and this seems really promising - I'm itching to get back and play with it some more, but already it feels closer to the tone I'm after.
I'm not very excited by the hockey puck or other elastic suggestions (as effclef said, interesting but perhaps not very practical). At sort of the other extreme, I'm hoping to find someone who can make me one out of granite - closer in mass to brass, but without the ringing. This is quite intriguing to me at the moment.
More later.
-Bob