Hello Tholt,
As told in a previous post, I strongly recommend you have a look at "Joe's tube lore" for good advices on 12au7 and 12AX7 tubes. I went through it and tried many of its recommendations with my Standard LE. Worth it. Just type it on Google and you will fall in Audio Asylum where it is based.
Now, the VAC tubes that come with the unit are....hmmm very so so. Flat, dull and no punch.They will give you a bad impression of your unit, period !
IMHO, Joe's....recommends RCA Clear top 12 in the 12AU7 spot and Amperex Bugle Boy. The RCA'S are very good overall performers and easy to get at a reasonable price. Got a couple of pairs myself and strongly recommend you choose those as a starter. Can't go wrong with those. The Bugle Boys are scarce and expensive. I've got Valvo ones supposedly made in the same factory, at the same time, for a fair price. I love those for the better dynamics and openness.
As for the 12AX7, there is a lot of choice and tried many combinations. My preferred ones are the 5751 like Joe's recommend. I love both the GE JG5751WA and the Sylvania JHS-5751 or Gold Brand. I prefer the GE for rock because of their slightly recessed soundstage with its better detailing of electric guitar. Bass and drums are tight and slammy with this one. I prefer the Sylvanias for Jazz, soft rock and ambient. Very warm and lush mids one with voices very upfront.
GE'S are common on Ebay and Sylvanias quite expensive but damn their worth it.
Good luck !
