see what happens when the amp inputs are shorted, this info might tell the manufacturer something useful when you talk to them. If you're not applying any signal (playing music) then there's no AC component to confuse the meter when you take this meassurement, which you do appear to be doing correctly. Any offset values >100mVDC I would really start being concerned, & the amp may even go into DC protect mode around that point.
How much DC is considered safe?
If you measure the DC voltage at the amp's speaker output terminals, how much DC is considered safe? I have a pair of monoblocks that I measured a maximum of -32mV on one amp, dropping to about -21mV on the other. Will this small amount of DC cause any harm to the speakers?