What causes an amp to discharge a thump.?

Well, I am not a technical person, but my guess is that it stores up some power that needs to dissipate before you turn off the preamp. Wait a bit more and see if it's not gone.

Your main caps are dumping at turn off. I have fixed that problem in amplifiers by using bleeder resistors to ground. It totally eliminated the thump but requires soldering and some circuit knowledge to do the job risk free... If you can overlook it, it is fine and shouldn't hurt anything. Several older amp designs I have come across do this but some newer ones do too. The best way to solve the problem, from the designer's standpoint, is to put in a relay and muting circuit - but that costs money.

Arthur

The thump is caused by a DC transient that gets through to the speakers. If the amp is direct coupled, meaning no capacitors in the coupling stages, then DC offset will be there. Also, the speakers, by presenting a reactive (rather than resistive) load on the amplifier, cause the amplifier to be slightly unstable, making conditions more ideal for the thump to be heard.

Usually amp manufacturers will design a "speaker protection" circuit which consists of either time delay solenoid switching or an output shunting relay to defeat the transient on startup and shutdown. Some mfrs think that protection circuitry and/or DC coupling capacitors degrade the signal path and opt to leave them out.

A call to Quad's tech support should be made just in case there are protection devices that may have failed. Just to be sure.