Let me be the first to attest to the use of 2 x 12au7 tubes in the LS100 phono stage. Previous to this, matched with the Benz Ace High MC (2.5mV) on a VPI Scout, the gain was VERY HIGH coming from the phono. While the sound was still very good overall, this gave me very little usable control over the volume in the phono stage (the sound was already very loud at about 7-8 o'clock. 9 o'clock was DAMN LOUD. And beyond that was...YIKES!). This was only really an issue when using the remote (one short click would bring it from inaudible to pretty loud), and when switching between inputs (dont forget to turn it down before you switch to phono!).
Switching to 2 x 12au7 tubes definitely decreased the gain somewhat. I still think its slightly higher than the line outs in my system, but much closer. Sound is still very good, but I'm still listening and evaluating. Overall, it seems like a pretty good alternative.
My questions for everyone here are:
(1) which tube posiitions in the phono stage would you consider more important to the sound (the original 12ax7 or the 12au7)? I have one NOS Mullard 12au7 and the stock 12au7 - I'd like to put the better tube in the more important position. I know I can easily try both, just curious if anyone has ideas to support observations.
(2) Any favorite 12au7 tubes to try out - new or NOS? If anything, I'm looking for a tight, accurate bass sound to counter a slight boominess from the bass coming from my phono setup.
Or maybe any accounts of using 5751 tubes as I mentioned in the previous post.