hi jim,
vmps are rarely found on the used market, but they're really inexpensive used - partly, in the case of the larger sub, that at 140lbs & >8cu/ft, it's expensive to ship, i'm sure. there was one on ebay recently that got no bids at a starting price of $275, i tink. ya mite be able to find it in the completed auctions page if ya search vmps on ebay. but, check some dealers for pricing - compared to the powered subs on the market, these are a great deal even bought brand-gnu, considering shipping is included in the price! :>)also, i highly recommend getting all the awailable upgrades
i am a firm believer in using a pair, set-up in proper stereo configuration on the outside of the monitors, but, if ya gotta do yust one, then in the middle is the best place, but it would be preferable in the near-field - i.e.: as a coffee-table. (ackshully, i believe that near-field placement would be best for a pair, too - like end-tables, but my current set-up precludes wires running across the middle of my room.) make sure ya cross 'em over wery low, w/a slope of at least 24db/octave especially if yer only gonna run one sub. that adcom, which puts out 325w into 4 ohms, would do fine w/a pair of subs, but i wanted to get the amps closer to the subs, so i run a pair bridged. at ~$400 used, these make great sub amps! ;~)
good luck, doug
ps - check out vmps' www - brian cheney, the owner, has lotsa inneresting stuff to say about his view of speaker design & sub design.