Never to be one to leave well enough alone, I re-wired the noval socket to suit a handful of near-NOS 6CG7's I won on eBay for rather cheap. Since the black wire that was already there wasn't quite long enough to reach pin 4, I re-did the heater wiring with some Belden shielded cable, and took pin 9 (shield) to the chassis ground. The result? I have a new favorite. Great soundstage, warm mids, and really sweet HF that reminds me of my SEX amp. This makes sense, as the small triode on the 6DN7 is essentially half of a 6SN7, and the 6CG7 is just a 6SN7 in a different-sized bottle.
I was afraid these might sound more "tubey" than the 12BH7 or RCA clear-top 12AU7 I had been using lately, but that's not been the case. The three RCA tubes I acquired look to be from the 1950's or very early 1960's, before the "meatball" logo disappeared. The have huge, dark-colored plates and there is a shield between sections, which was taken to ground. My understanding is that the shields were added due to the sections being shoved so close together compared to the octal 6SN7, increasing the chances of crosstalk between channels.
I also stuck a 6922EH in there for kicks, since the operating point seems similar to that of the Seduction (e.g., Agilent HLMP-6000 LED bias (~1.56V) and about 75V on the plates). The sound was definitely more clinical and "modern," in addition to a smaller soundstage than the aforementioned 6CG7. Not a fan. I've found this to be the case anytime a higher-mu tube is added, such as the E80cc. The extra gain makes for a more forward (and I'd argue, fatiguing) presentation.