I wanted to revive this thread just to thank everyone for the great advice and provide an update. The depth of knowledge and helpfulness here on Audiogon is truly humbling.
After exhausting my possibilities with placement and room treatment, I broke down and bought a pair of Epos ELS-3s with 30" stands. As expected, they are a much better match for the small room, pretty much eliminating the boominess and mud I was getting from the Advents. At that point, I had changed nothing else in my system.
What I was NOT expecting, was the incredible increase in detail and speed. While the Advents were close to SOTA in the late 70s, time and technology march on. I just hadn't realized how slow the tweeters in the Advents had become, and how much faster the 5.5" mid in the Epos could move compared to the 10" driver in the Advents. The difference was phenomenal, especially in my small room, and even more so at low listening levels.
With the NAD 7140 powering them, I certainly wasn't getting the bottom octave or so that I got from the Advents, but what I got was so much cleaner I almost didn't miss it. Since then, I've added a Hafler DH-200 that I'm about halfway through modding, still using the phono/tuner/preamp in the NAD and now I'm not missing it at all. The added power and damping factor of the Hafler has improved the bass of the Epos' to the point where I'm giving my room just about as much as it can handle without overloading it. At low listening levels, I still get almost everything. I'm amazed, frankly.
I also built myself some speaker cables from nice 12 gauge OFC and gold spades and bananas. I'm stunned at the improvements I've gained just from these simple upgrades, and for about $400 total shipped. Next I'm ready to start modding out my Thorens TD 150, then probably a second round of mods on the Hafler. I'll be hitting you guys up for advice again, I'm sure.
Thanks to all who have contributed so far. Have I reached the stage yet where I can post my system without embarassing myself? :-)
David