One channel missing HF in small SE power amp

I finally got my small SE amp with one 12AX7 and one 6BQ5 per channel separated from tone and balance network and rigged up as a power amp with a passive pre.

Now, I'm puzzled to find that one channel is missing its HF. It sounds as if the tweeter on one speaker is out. It's not the tubes, as I've checked with good replacements. I hadn't noticed this as I was using unmatched junk speakers for testing. But I've tried it now with two good pairs and the results are the same. It also sounds out of phase!

This is a new fault to me. Can anyone please tell me where to look? Could it even be the transformer? The amp has quite a small component count and I've checked all values and connections for touching wires, etc. I've removed the tone, balance and volume pot from each channel and the signal now comes in direct to the grids of the 12AX7. I attach the circuit.

Any thoughts, or similar experiences?

Well, here's the thing. When I disconnect the feedback, the treble returns to both channels (quite bright in fact) and the stereo image snaps into place. Clearly my feedback arrangements are not right yet. There are not many original passive components left in this amp. Maybe it's time for a complete rebuild, with a more well known circuit? Sounds stable and pretty amazing as it is at the moment; just shows no mercy to the harsher end of CDs. Ah well, at least I know the transformers are fine.

FWIW, I use metal film Rs and Silver Micah caps for feedack with good results I think.

Thanks Jay. Just pondering where to go next, while Steely Dan sounds pretty good with the amp as is! Most of the day was spent fixing a leaking dishwasher...

You can always use a minature pot for the feedback R. I do in my ST35 clone.