I had two of them (still own my 1st one).
I was surprises when the 2nd one didn't sound anything like the 1st one, and quickly sold it off.
I still have the stock "Marshall" labeled Sovteks in mine.
Haven't had a need to change them yet...
I would probably try JJ's next? i dunno...when it comes to KT88's (which I have 3 amps that use them), I actually have no real idea...
I think I have Winged C's in my Twin Jet...
I think, anyways....Reinhold put them in but I don't remember...