typical fare for those amps - I've got one myself, they sound shitee in stock form. however, the transformers are really nice quality, so rewiring the thing results in a very nice amp indeed.
I'd really not bother trying to tweak it - the preamp isn't geared towards good guitar tones and the filter caps are old and tired. Strip it down to the bare chassis and transformers then start again.
You've got a nice pair of yellow-label Mullard EL34s in there, so you've got a headstart in the tone stakes right there. the two ECC83s and the EF86 mean you've got plenty options to work with - a pentode (EF86) front end sounds very nice indeed as you'll know if you've ever played a Matchless DC30. You also have a GZ34, so some tasty rectifier sag is available too.
You could build a crackin' JTM45 from that - perfect platform (better than the Kelly PA amp you have)
