Amp Oscillation and Negative Feedback?

[Jerry2013]

I have an old early 70's 2xEL84 Traynor Bassmate that sounds great, but the design has no negative feedback oscialtes anytime the treble is turned up to over 7 or so. Problem is this amp oft sounds best with the treble up high, and if I turn the volume down so there is no oscialltion, it isn't loud enough for me...I like it plenty loud, and its only a 25 watt design, IIRC.
The design of the amp has some great tube sag, and a lot of touch responsiveness, as many old designs do, and I love that vintage tweed type "feeL'.
Question is, can negative feedback be introduced to a small degree, to retain most of that old tube amp responsiveness, and tame the oscialtion ( sorry, can't spell that werd), and to what degree ,without ruining that "sag"?
Live with it?

[Blackburn]

You can probably fix the oscillation without introducing NF. Just gotta find it.