I like a super firm mattress, due to back injuries in the past. But hey that's just me. Given that I can't fathom buying a mattress without trying it first in a showroom. Or I suppose a written statement that if I don't like it the seller comes and picks it up no questions asked or expense to me.
Yea but...and there is always a but.....it does it properly. It does not just short the two together. It uses an active combining network by utilizing the summing junction of an operational amplifier.
Yes connect the sub via the pre outs in #11 above using two rca cables. If you hook just one of the pre outs to the wireless transmitter you won't get the other channel's low freq info. The sub has a combiner at the two inputs to make a mono signal for the sub amp. Use the freq control "low pass" knob to cut the high freq info to the sub. Page 8 of sub manual.
You will have low freq coming out of both your mains and the sub, adjust level of sub to taste.
Do not use a Y cable to tie your 2 pre outs together.
Emile, follow Chris's instructions and recommendations. That's my advice to you. After all the K510 and 691 are the hi freq section of the KPT-396s. They do not sound thin and tinny.