The new tubes came in the mail Monday, and I've been anxiously awaiting the opportunity to try them. I finally got some free time tonight and swapped in the RCA clear top 12UA7 and removed the jumper that I'd put in from A3 to A8.
As soon as I powered up to do the voltage checks, I knew something was different. The heater on the 12AU7 was much stronger than before. All voltages checks were good. I tested the tip voltage and that stayed at zero. So, hook up the DAC and listen to some tunes!
I'm now happily enjoying some Van Morrison and wondering how long it'll take me to go through and re-listen to my entire collection. The clarity is amazing and the background is absolutely black (without the Speedball). I've got the Speedball built, and will probably swap it in after a week or two.
Mark this one solved! Thanks Paul.