(Smiley bows at your feet.)

 

There aren't any other sound issues occurring, amp is quiet, its just it sounds like it has a blanket over it tonally, the highs are not very present.

Are you sure it's a guitar amp and not a bass guitar amp? The bass amp I have sounds exactly as you've described, when I try to use it for my Strat.

 

I thought it might be a bass amp too, but it's got reverb. My P-bass doesn't sound as good as a Strat does with reverb

Any idea how old the tubes are?

Are they microphonic? Do any of them make crunching noises if you tap on them?

Found this:
http://en.wikipedia.org/wiki/Teisco

 

Are VR1 through VR4 labeled? On the schematic, the way they are stepped with capacitors, seems like they could be some sort of tone control.

 

There is also the possibility that the "bright" switch has gone bad, and the fact that guitar speaker(s) usually fall off at about 5KHz to 7KHz and this one might be bad. Try another speaker before spending hours and hours chasing the circuits!

The 47pf on VR1 is a high frequency carrier, but they are usually bulletproof ceramics. Same with the 240 pfs on the tone stack (Vr 2,3,4).

ps, the brite switch is the one on the top of the tone stack.