Since no on else is hazarding a ‘guess’, here goes nothing. As long as the bias potentiometer has enough range, it should work just fine. However, it will still likely be pretty clean. I don’t have much experience with the KT90, but the SVT itself is designed to be pretty clean. Depending on the speaker cabinet you have, this may end up being a clean amp no matter what. Which means an overdrive pedal is the only option.

I’ve learned soundmen don’t like working, period. I’ve tried bringing mics if I just bought a cool ‘vintage’ piece. You’d think they’d love the opportunity to try a real mic, as opposed to some modern Shure Beta ‘ho-hum’ mic. Nooooo. That would mean sitting there and mixing and eq-ing.
I digress. If your one SVT is ‘nasty’ and you want it clean, something may be wrong with the ‘nasty’ SVT. Maybe one tube is bad, maybe the bias is way off, maybe it’s time for the 50,000 mile check up. They should sound similar. Have a tech scope it our, and see how much it’s putting out and how clean the output is. You may be in for a shock. Just a suggestion.

I don’t mean to slag your tech, but something is different between the two heads. If this were me, I’d do the simple and check the bias in each. Then swap output tubes between heads. If this doesn’t prove anything, I’d swap preamp tubes between the two heads. Then, as a last ‘test’ because it takes sophisticated test equipment and requires real knowledge, check the B+ and the primary impedance of the output transformer. Something will be different, and that something can be slightly corrected and/or compensated for if you know what you are doing.