i've had success using my marchand/vmps system as high as 100 hz (they're presently set at 60hz), and this allows the main speakers relief of the need to play that last octave or so, making them all the more transparent & detailed.
i'm sure that stentor is a lovely sub, but at $4k usa prices, it is just not competitive, imo. i know these are better deals in the uk. i am a notorious cheapskate, but when it came to subs, i bought brand-new: both vmps larger subs, *and* the marchand x-over. one adcom gfa555 was bought new years ago & pressed into service, the other i bought used to have a matched pair. the total expenditure, including both amps, was still <$2.4k... if you went w/the vmps original subs and a single stereo amp (all that's needed, really, except for *really* big rooms), you could bring it in for ~$1500. that's for *two* subs. you could even save more money, as both the vmps & the marchand are available as kits...
gunbunny, while powered subs may seem less complicated to set-up, in the long run, separate sub/amp/x-over ultimately gives you more flexibility, imo, but hey, it's your call! :>) as far as two vs one, i *strongly* encourage you to get two, especially since audio is a priority. better soundstaging, easier integration w/room *and* monitors, less thd at any given spl, w/two subs. as far as getting a pair of *lesser* subs, you may want to consider getting a better single sub now, and adding a second, as funds permit.
regards to all, doug s.