AA764 Champ Farts Out, help!

[sixstringer]

1966 Fender black-face Champ (non-vibrato). All new electrolytic caps, new 8" MOD speaker.
The only mod is a tone stack bypass switch. The circuit is stock with the TSB turned off. The amp farts out significantly at medium and above volume with/without the TBS engaged.
Tubes are original by test good. What should I check next? How do I resolve the issue?
Thanks in advance.

[bepone]

coupling cap to the output tube leaking? to be replaced?

[Colossal]

That is what those amps do. The best Champ is the one that is never played above 2-3 on the Volume.

Solutions:

1) Change both coupling caps to 10nF (from 22nF) and if the second preamp cathode is 1k5/22uF, lower that cap to 1-4u7
2) Install a 12" speaker
3) Both 1 and 2

There's more that could be done, but then it's just easier and better to sell it, unmolested, and build an amp that fixes everything you hate and sounds killer.

[Stevem]

With the tone stack engaged your overdriving the output tube too hard with low end.

With the tone stack out of the circuit your very much overdriving the output stage, but with too much overall signal.

If with the tone stack engaged and your on your bridge pickup the amp should not be farting out too bad unless that pickup is just plain too close to the strings!

[sixstringer]

Turns out it was the Tubes after all. Replaced the old tubes with new and now it should like a Champ. And the TSB switch adds a nice crunch.
Thanks, everyone for your help and advice. I'm going to try the cap value adjustments too.