I had not heard any popping from Geoff's 2325 so as to be able to pinpoint it. I've had issues with the small buffer board about in the center of the bottom of the chassis so always rebuild this with not only new electrolytics, but replace the double diode with two 1N4148's in series and replace all of the transistors. I will post the subs used later when I'm at home to access the info.
Being too lazy to go back and look, has anyone suggested connecting the left preamp output to the right main amp input and vice versa on the other channel to isolate where the fault is, amp module or preamp side? If the noise is still on the same channel it's the amp. If the noise goes away it may be that the switching contacts on the preamp out jacks are dirty. They can be permanently bypassed by the use of short RCA to RCA cables. Do not use any unshielded "u bars" here.