connecting a sub to a 2-channel system

The loading presented by the sub to the power amplifier is not the reason. To prove that, see what happens when the sub is connected but its volume setting is turned all the way down.

What you are perceiving could be the result of a phasing problem, perhaps in combination with too high a crossover frequency, or a placement problem, or a room reflection issue. Given that the sub only provides two possible phase settings (0 and 180 degrees), and you've probably already tried both, my guess is that it's a placement issue, which at some frequencies in the bass region is resulting in the output of the sub arriving at the listening position with improper phasing relative to the same frequencies arriving from the Quads, for both settings of the phase switch.

Best regards,
-- Al

I'll try it tomorrow night with the sub turned off, see if that restores the Quads' ruler-flat mid-range. I feel like I am missing some detail in the mids, like the sub is covering it up or something. This is only temporary anyway, I miss the sub in my home theater too much! Funny how I used to crave bass when I first got into this hobby, now I just...don't!

I think I know what was wrong, crossover was not set correctly, now the sub and speakers overlap pretty seamlessly. Thanks all.

OK now that my sister has left, her visit ended, I returned the sub to the home theater system for our viewing of "snow white and the huntsman." I listened to my 2-channel system for about 2 hours last night and I think I miss the sub. It added some weight and presence to the music. Another thing: I use a computer desk in my listening room to work from home, I just move it out when people come in for a listen. I had to move it back in last night. There is a large 24" monitor that sits on this desk, and I think it really kills the imaging! Grr!