Not just volume pot tolerance - one might be linear taper, the other log taper. That alone could account for a large disparity between loudness & volume pot settings.
Also - in years when 12DW7 tubes & their equivalents were scarce to nonobtainium, many SVT repairs swapped in 12AX7 instead, and that can account for big differences in gain. Some amps were modded to accept the AX's, others weren't. And some of those modded to accept the AX's later had DW's or maybe newer JJ ECC832 installed but did not get modded back to original. Those & other opportunities for gain hijinks might explain why one amp is one way, & the other seems different.
I'd look at the "small" tubes first, make sure they're the right ones in the right places. And check the circuits for mods made &/or unmade. And the volume controls for log/linear.
Also keep in mind old played-out power tubes sound mushy and suffer reduced gain & power delivery when really worn to a frazzle though they may keep running. One SVT I examined years ago had only one out of each triplet of power tubes still operating, you can imagine how lousy it sounded though it still "worked." Because of the impedance mismatch with only one frazzled tube operating on each side, it turned out a very distorted 45 watts into its 8x10 cab. Miles off from the typical 240-260W one sees with a fresh sextet of 6550 or KT88.
Output transformers don't lose power when "old and worn out" so that's one less worry. I'd expect an OT with shorted turns would be a sure fire fuse popper. Can't say I've ever seen an SVT with any bad transformers. That's not to say it's impossible, just not likely. There's lots of other places as you well know, to have SVT's go off the rails. You'll figure it out, I'm sure.