The presence circuit is a pretty simple and old school design. Describing the amp as sounding "tinny" doesn't make think of an amp with more presence. Do you remember the amp sounding tinny with the presence control turned up? Now if your feedback resistor were shorted that might make the amp sound weak and thin. But resistors rarely fail short. They usually fail open. If the presence control is working I wouldn't suspect that circuit for now. If it's not and you think presence (in general) sounds tinny then I guess you could suspect the presence circuit. Though as with resistors, pots usually fail open.
The schematic looks to me that U1a before the preamp out (so that's not it) and U1b isn't in the circuit unless the power amp in jack is used. Does your amp sound tinny when the power amp in jack is not used? If it does then U1 isn't the problem either.
But other than those two circuits there isn't much to suspect, as you noticed. I suppose you could try another tube in the phase inverter before cracking it open. If that doesn't help you're down to bench testing for voltages and the maybe the OT.
P.S. Stop disconnecting the speakers to quiet yours and your friends amplifier. The preamp out jack doesn't interrupt the signal so your power amp is still trying to make watts when it's in use. Disconnecting the speaker is disconnecting the load from a working power amp. That is a very bad thing to do to a tube amplifier and may detriment very expensive parts like the output transformer. If you have been doing this to your own amp for recording and such it might explain why you have a problem. If you keep doing it to your friends amp you may both have a problem soon.