If you plan to use it for Home Theater and want those depth charges to kick your adrenaline in, get something like the SVS 3000 series. If you go this route, I would start a new thread under the subwoofer forum to get assistance finding the right sub for your situation.
If you really only care about music, your choice is not quite so obvious. The graph I posted for the ICS 300 is pretty impressive and assuming that your next revision sub was not a step backwards, it seems a pretty fine sub for music (unless you are a pipe organ aficionado or into modern music with ultra-low synthesized bass). For reference, a standard kick drum is at 50Hz, the bass has a lowest fundamental of right around 40Hz with some non-fundamental artifacts down to around 30Hz. Generally, in this situation I might recommend the Hsu ULS-15mk2 as a good performer without breaking the bank (but we need to know how big your room is before we can really say what sub you should get). But the Hsu will cost more than $550 and there is something to be said for having matching aesthetics and a "thrift store find" Snell/Arcam system.
Definitely gently poke at the surround for the driver of the subwoofer to make sure it is not ready to crumble, and since you have the plate off of the back, post the best pic or two that you can get of the driver - some of the guys here may be able to give you an idea of the quality of the driver. Typically, most subs made by speaker manufacturers are not very good, but Snell definitely was not a "typical " company, so it may be a sub well worth reviving!
Last, don't hesitate to call any of the mainstream ID sub companies: SVS, PSA, Hsu, Rythmik. These companies all have people to help you pick your best sub option (based on room and listening habits) and see what option each suggests. It is a free education on what factors matter and these guys generally offer quite a bit of expertise in formulating your best solution. You will get a pretty decent education just from talking with these guys, and the process may give you better insight into your choice.
But again, probably best to start another thread under the subwoofer forum to focus on your sub choices with the best people to discuss it with.
Oh, to answer your other question - No! You generally only want to listen to stereo music on two speakers (or two speakers and a sub). The technologies to synthesize extra channels from a stereo signal generally don't work well - I have heard nice things from them in places, but not consistent enough to actually want to leave it engaged. Even so, that is surround channels, more than the center.
There are some quality recordings made in 5.1, but you have to seek them out and they are far from establishing there place as the future of music!
