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.
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.