Ive owned my Bel Canto SETi 40 since 2008, & absolutely love the amp. I too use Shuguang 845T's, the metal plates sound very open, but can overheat and malfunction, I have a few pairs in stock so I don't fret over this much, and have some B's in stock as well for back up - the B's are hardier, but to my ears don't sound as sweet.
Going back to your original question -I'd try different 12AX7's - and see what sounds best to your ears and in your system - I once tried some Chinese rebranded tubes bought from Guitar Center - "Groove Tubes Gold" series that sounded warm and really really nice (pleasant surprise, maybe not neutral though), I've also experienced some NOS RCA's that were OK, some Sylvania 5751's that sounded less bright (boring), and currently have some Psvane 12AX7-T's that sound rather neutral to my ears in my current set up. This may be an interesting thread, I'm keen to see what fellow owners have to say.
Going back to your original question -I'd try different 12AX7's - and see what sounds best to your ears and in your system - I once tried some Chinese rebranded tubes bought from Guitar Center - "Groove Tubes Gold" series that sounded warm and really really nice (pleasant surprise, maybe not neutral though), I've also experienced some NOS RCA's that were OK, some Sylvania 5751's that sounded less bright (boring), and currently have some Psvane 12AX7-T's that sound rather neutral to my ears in my current set up. This may be an interesting thread, I'm keen to see what fellow owners have to say.