Thanks for the advice! A few thoughts:
a) I don't think anything in my equipment chain should be problematic with horns, though my living room is another story entirely. For playback, I have a classic Marantz CD-67SE, which is a very warm-sounding CD player. I also have a Cambridge Audio DAC that I run an Airport Express through, using Apple Lossless files. On the analog end, I have a Denon DM-500M turntable (a 2007 model w/ huge cherrywood plinth), AT 120E cart with microline stylus (the one for the other AT cart that escapes me at the moment). I have my Seduction using 6922 tubes and C4S loads on them, going through 1M of interconnects to my SEX amp, which are also C4S'ed. I'm using 6DN7 tubes with stock coupling and interstage caps and Specos wired on a DPDT switch for 4 and 8 ohms for my OTPS. The MQ upgrade seems enticing, but expensive. I'd rather drop the $400 or so I'd spend on new iron and caps for speakers!
b) I think I know my problem with my current Klipsches (and the RF-5s of yore): Klipsch speakers, at least ones that use bass-reflex loading, seem to have huge impedance dips and nasty phase angles in the midbass region, between 85Hz and 250Hz or so. The RF-5's in fact dropped down to something like 2.5ohms, which is lower than the reflected impedance of the 6DN7 tubes, even with the 4-ohm taps. As a result, there was too much interaction with high-impedance of the S.E.X., leading to strident-sounding highs and mids at 8ohms, and very feeble bass at 4ohms (bass at 8 ohms was louder, but very flabby due to the damping factor being so low). Note, I got rid of the RF-5's before I put the current sources on the plates of the smaller triodes. My current Klispch Icon W14's are merely OK sounding, but seem to be too prominent in the mids and highs, and bass isn't great considering the smallish 4.5" midwoofers. The F0 is probably something like 85Hz!
I have been planning to put in the 6EM7's (since I have indeed gone away from using a resistor as my plate loads) and change R1 to 1000ohms. I currently have tons of power using the 6DN7's - that aside, is it reasonable to assume that the impedance issue might be less significant with the 6EM7's? Also, the living room the speakers are in is less-than ideal: 10-foot cielings and wood floors. The bookshelf speakers are on the fireplace mantle, which corner loads them and gives a bit better bass, but they are too high and have no seperation or image. That's why I've been considering the Heresy III.
In all, can I assume that the Heresy III will sound better than my Icons and perhaps even better than the current-hungry RF-5s? I want a classic sound with a ton of presence, especially since I mostly listen to acoustic jazz. Would the Heresys be nicer to my amp in terms of impedance dips (they are sealed and the woofers probably don't have a huge excursion)?