Basically the same question asked in this thread:
Replacing a 12ax7, Hartke LH 1000
But the Warwick Quad VI runs basically a full tube amp into a Mosfet power amp (I think, read it in a magazine article, link to article upon request).
I would think that this would mean the answers given in the linked thread might be different.
And any tube suggestions for a guy who idolizes Doug Johns' tone?
