That is correct, it is the P5xe. I'm not quite as ignorant as it may seem; I couldn't layout or interpret a tube or a solid state circuit if my life depended upon it but I do know what sounds good...I'm definitely not a technician so before I plug something new into my $6,700 Hovland (the very best preamp that I have owned in 30 years) I want to be positive that I am not missing something important...
You're right, variances in the 12AX7 (and all tubes in general) can make a huge difference in the quality of sound. I've tried several sets of Mullard's with varying success. The first set I tried was a definite improvement over the stock factory tubes; the second set of Mullards were terrible (very noisy after a few months and both sets came from UpScale Audio). I'm back to the stock tubes and they sound great.
I educate myself with my ear :), and through interactive mediums like Audiogon. If I like the way it sounds I go for it. The purpose of the exercise is to compare several external phono's to the Hovland's internal realizing that ultimately they all pass through the line stage. Next up is probably the Audio Research PH7. The new Ref 2 phono will follow. And then , a combo of the Ref 5 preamp with the Ref 2 phono. It will be interesting (for me at least) to hear how the Hovland compares to these other combos...