tape out = preout ???

Looking at the manual for your integrated it specifically shows a diagram using the pre-outs to drive a sub-woofer while driving the main speakers from the speaker outputs (not pre-outs). If that does not work I would contact your dealer or Cambridge Audio. If the hookup is correct (it probably is) then there may be something wrong with the unit. What you are trying to do should work according the manual.

I see that the cambridge audio 840a v2 is an intergrated amp. therefore, the mains get hooked up to the speaker terminals and the sub get hooked up to the preouts - this is assuming that your sub is an active sub and not passive.

yes it is an active sub.

i talked with audiopluss services and they said not to use the tape out because, as folks above have indicated, you can't control the volume. they said to take the top off the amp (after unplugging and letting it sit over night) and then see if there are loose connections. if the preouts still don't work, i'll send it in for warranty repair.