Getting a particular T2 stable can be a bit of a game. The problem arises from the fact that the feedback trace is quite long, physically. It was less long in the original Stax T2 because it was more compact (and as a result ran incendiary hot). Anyhow, the feedback trace has to get all the way from the output tube's anodes/plates all the way back to the input tube's cathodes. Mine used to hoot too - you need to adjust the 15pF value in small increments both upwards and downwards, and possibly add one or both 2.2pF caps, and again be prepared to tweak the value. While looking for oscillation and its frequency. Once it is stable, do a frequency response sweep and look for a response peak - if you find one continue adjusting those caps until it is critically damped.
It is interesting to note that 15pF and 100k gives a dominant pole at ~100kHz whereas the oscillation is at 685kHz, which implies that there is a secondary feedback loop, possibly inductively coupling into the feedback trace.
I have a vague recollection that I cut a trace and re-routed in some way, or changed the position of the upper feedback resistor, but I'd have to tear my T2 apart to find out what I did.