If R13 is (56k) then it's likely done to tame the amp down as the two SS dirt stages would be too much for the Amp section.

Normally R13 would be at least 220k,,more common value is 1Meg.
With 1meg at R13 will open the amp up huge but you will then have to re eq the whole amp circuit.

You can play with a few sections;
Remove C51 for a bit more top end, if you still want more top remove C50 as well but watch out for feedback at high volume.
If R72 is 500k you can add a treble bleed around it ~470pF is common.
(just go find all the classic valve amps for clues)
You can fiddle with the values of R18 & R25,, dig up some bassman/Marshall schematics for ideas. That changes how the PI distorts amoung other things.
If too much bass make C10 smaller ,try 1n5.
Other stuff;
You don't need metal film for plate resistors normal carbon is fine.
you use metal film for signal path not DC stuff.
As to your ideas of the SS crap in front,, it's anyones guess how it will help,, frankly the whole thing is a bit of a miss mash.
I'd focus on getting the Valve Amp section to work well then down the track you might be able to tweak the TS part.
I'd bypass the the gain section around u3A as it's likely too much,, hence they had to turn down V1 (R13 56k)
With a pair of 6L6 running from 560VDC would be way over 30Watts so it should be loud if not then they don't know how to build Valve circuits.

Phil.