Help with Sound City B150 feedback circuit

[01redcobra]

I have a SC B150 amp in here that has some instability issues. Somewhere in its life, someone replaced the output transformer with a Heyboer replacement. I can see that they had some issues with the amp because there are all kinds of mods done to deal with instability. And obviously they didn't help.
What I noticed was that the Heyboer does not have the feedback winding as on the original transformer. The loop is now attached to the 16ohm winding of the Heyboer with the same 10K/4.7K divider that was originally used with the feedback winding.
Is there a chance that this could be the cause of the problem I'm having with the amp? Cropped schematic attached.SCB150crop.pdf

[g1]

Have you tried disconnecting the feedback loop? It should make the problem worse. If not, the phasing of the OT may be reversed.
I would think someone would have checked that before modding for instability, but you never can tell.