Kevin, here's my latest plan, which I think pretty well follows along with your last suggestion. I would connect the RCA outputs of the S300 amp to one of the RCA inputs on the switch box. Then, the RCA outs of the switch box would go the high level inputs of the subs, one channel to one sub, and the other to the other sub. The sub-outs of the Denon 4300, which would only be turned on for HT, would go directly to the LFE inputs on the subs. Thus, when I want to listen to stereo only, I would switch to the input on the switch box from the S300 amp, and for HT, switch the box to an unused input, thus cutting out the input from the S300, and turn on the Denon so the LFE sub-outs then input to the subs. This also lets me get back to Audessey control of the subs for HT. None of my speakers are connected thru the subs, so there is no problem there. In addition, the RCA box has a remote, so I can incorporate control into my Logitech Harmony Elite universal remote to switch the box to the proper input remotely. I'm getting the switch box Thurs and will hook up this scenario to see how works.
Thanks for your input.