The thump is caused by a dc voltage offset (as you said dc accross the speaker). This offset is usually caused by the leakage of electrolytic capacitors accross the voltage reference diodes on the driver board. The capacitors are 25volts, 220mf, cost about 79 cents a piece (there are four of them to replace). However, when this happens, usually the reference diodes (6.9 volt temp compensated Zeners) or the bias control transistors (MPS-A63 and MPS-A13) also need replacement. Your technician is full of it, the power supply capacitors are very durable, and most likely are not the problem. By the way, the power supply capacitors, 120 volts, 36000mf, are very expensive, about $90 a piece, and are difficult to find. Your best solution might be to search on the web, e.g. Ebay, for Adcom repair services, which can cost you a couple of hundred bucks per amplifier.