Hello Mdrew,
Hope you are well. Instead of hooking speaker cables up to the new sub, use the pre-outs that the H/K 7200 has for the channels for the Zone 2. Always cleaner to use the pre-outs.
As far as tone controls you could stick a good equalizer in-line with the sub. That only solves the bass control though. To get treble control would be harder, you would have to hook a 2 channel amp to the line outs of the subwoofer, then hook your speakers to the new amp. All tone control would utilize the EQ and would be passed along to the mains through the subs line-outs via the new 2 channel amp. Probably a heck of a lot cheaper to get a 2 channel amp and pre-amp on ebay, than this elaborate work around though, hehe.
If you just want bass control, and can live without treble control, try to get a subwoofer with a built in parametric EQ. Lot less equipment to buy with this option. The SVS PC-Ultra has built in parametric EQ, it is $1,149 + shipping, and they don't get much better.
Later,
TonyM
