The S10 would probably need a 100-120Hz crossover point, so the sub would be required to be in the front of the room, near the speakers. The sub would need to be capable up to about 140-150Hz, to provide a clean crossover range (it's not a wall, it's a slope). The S15 is a better speaker, in that it has quite a bit more mid-bass, allowing you to cross over at 80Hz, giving you sub placement options other than just the front of the room (Anything above 80Hz produced by the sub can lead to what's called sub-localization, where the sub stands out and does not integrate well with the main speakers). The tweeter is the same, but asked to do less in the S10 (because it's crossed over at a higher point, which may account for the cleaner treble you allude to).
The ML sub recommended above would be a good starting point, as it is currently priced at less than half it's original cost. Good quality sub, that can handle upper-bass well, while not becoming a one-note wonder, as so many cheap subs do. Also, it is a reasonable size, as many cheap subs are quite a bit larger (but poorer performers, appealing to the "bigger is better" crowd). It has 29Hz low end extension, so you can hear all of what Willie is doing on that Upright Bass!
BTW, sub placement is crucial to getting good sound, so have more than one placement option available.