looking at the circuit on this subwoofer, the preamp board has stopped preamping, most likely because of heat damage - the preamp power supply was 2x 2k2Ω 1W heaters shunted by 2x 15V quarter watt zeners.
Input RCA jack is connected via two resistors directly to a TL084 input. so far I can see a low pass filter and what looks like a buffer. output of the buffer goes to a front panel pot somewhere, either crossover or volume.
after this stage there's a standby detection circuit and phase control knob. the standby should power the sub off when there's no signal, but it doesn't appear to do anything apart from turn the standby led on and off; the output amp is stays fully powered.
have started rebuilding the preamp on the bench, pics to come
