Oscillation~ Pre or Power's fault?

[hajame]

A few months ago I encountered something strange with my amplifiers.

One day I when changed the interconnects between my preamp and power amp, a very loud mid-frequency tone came out from the speakers. I switch off the power amp late, 3 seconds, after that as I was too scared. I believed it should be the interconnects that triggered oscillation. So I change back to the original one. To my expectation, no oscillation this time... But something more serious....One of the tweeters was dead quiet!!!!! Burnt....Oh Dear!

After the tweeter was replaced, I tried those devil interconnects once more. To my expectation, oscillation occurred!! I suspected they were highly capacitive. Then I measured the capacitance of 3 pairs of interconnects:

A- (200pF) Stable, sounds the best
B- (160pF) Triggers very loud oscillation
C- (70pF) Stable, sounds a bit too bright

That's strange~ The most capacitive one was the best, and it's stable too!!

I am still muddled... Post here to find out which part I should blame.
Which amp was actually oscillating, pre or power? And why would the medium capacitive pair introduce oscillation?

 

[dangus]

Try a series resistor in the preamp outputs, maybe 47 to 100 ohms. I've seen that suggested with some op-amps to improve stability. (I need to add those to a line driver I built which also oscillates with long cables, but at inaudible frequencies).