Heres something I wrote in reply to another post here on ecoustics talking about why using a two channel amp (with seperate gains) or four channel (with seperate gains) for that matter could be used with a dual subwoofer setup (or more) and give you more flexabilty
this brings me back to an idea i shared on ecoustics before...if you had an amp like i do with seperate gains one for each channel you could use a fmod or seperate electronic crossovers to filter at different frequency ranges say one at sub range and another at midbass but this would require a bandpass filtering effect the way im thinking of it now....the advantage to approaching the setup the way im saying is that MOST amps are stable at a lower ohm in stereo than they are in mono so you have more wiring options that way if you need it plus you can use one amp to do what would have taken two amps to do normally
well for one you wont have to buy a second amp to have more flexability with say a two subwoofer setup say one for the low end and one set up for midbass that way each channel putting out say 80 rms per channel can be devoted to sub frequencies and the other can be devoted to midbass frequencies ...i did however realize a gain is not a power nob its to set the line levels but designing a bandpass circuit for say the one channel driving the midbass driver for any range you choose while maintaining sound quality aspects...now think even further outside the box and think of possibilities for tri mod options as well because even though some amps dont claim tri mode ability you can set them up to do so