Oscillation and Negative Feedback

I have been trying to tweak the Rotel RB991 amplifier, after getting some help from forum members in getting the schematic. I found that this amp is on the verge of oscillation and was probably found to be so even by the manufacturer, in that the usual bypass plastic film capacitors of the power rails on the amp board have been provided for but not populated as well as indicated in the schematic without values shown.

Given this, I set about doing the following mods:

1) Removed the 1000uf100v caps for supply decoupling on the amp board and replaced with 10uf100v polystyrene.

2) Added additional noise filter with 2E2 and 130nF in series between supply lines and ground.

3) Replaced output speaker cable with CAT5 braided cable.

4) Added 10pf silver mica cap in shunt with negative feedback resistor of 15K4.

The first 3 steps, led to a significant improvement in the sonics of the amp.

However, the last one has turned the amp into an oscillator, blowing the 6.3Amp fuses in the supply lines, as soon as power is applied.

My guess is that the HF fed in by the cap is providing a path for positive feedback rather than negative.

My question is, whether I can reduce the feedback resistor from 15K4 to around 12K in order to improve the stability of the amp (ofcourse, the 10pf cap should be removed in this case)??