hi howard,
before i got my subs, i heard the hsu's (older wersions - shorter & fatter), and various velodynes, including their biggest 18" model. (realize these were also older wersions - i got my subs a while ago!

in fact, my subs are the older upright style. megawoofers weren't invented yet, either; i have since upgraded my drivers to the megawoofers. as you may have figured out, i have a pair of these!

)
none of the above-mentioned subs compared w/the bass of big full-range vmps i heard at the time. the vmps' had basically the same bass component of their larger subs. and, my subwoofer rig, including amplification & x-over, cost less than *one* top-line velodyne.
for amplification, i originally was using a bridged pair of original adcom gfa555's, then i found they sounded better unbridged, yust using one channel of each amp for each sub. then, vmps' brian cheney sent me a pair of binding posts, so i split the two drivers in each sub to go to separate binding posts, & ran each stereo channel to each of the subs' two active drivers. they are now operating at 8 ohms, instead of 4 ohms... several months ago, i found a second electrocompaniet aw75dmb to match one i awreddy had; i am now using these in place of the adcoms. i sold one adcom, the other went in a secondary system, where the aw75 prewiously resided.
i am using a marchand xm-9LL active x-over, w/a 60hz x-over frequency. i have successfully used 60-125, depending on amplification/monitors used. 100hz min was needed, for example, when i had the 8wpc asl wave-8's driving my monitors. at their 88db/1w/1m efficiency, 8w yust wasn't enuff to drive 'em below 100hz in my big room.
re: the criterions, i was going to use a 80hz x-over point for them, cuz they don't go quite as low as my monitors, but danny said 60hz would be perfect, so i kept the marchand at 60hz. and, it *was* fine - seamless blend...
if you go into the audiocircle gallery, you can find pics of my room, & equipment set-up, if this helps put things in perspective. it's my opinion that, regardless of room size, best results will be w/a pair of subs. if only one sub is practical, then it should be placed *directly* between the monitors, nearfield being best, crossed over as low as possibble... and, i also feel using a passive sub w/outboard active x-over & amplification is best - relieving your monitors of those lowest frequencies is a good thing, & powered subs' electronics do not allow this to be done in a transparent-enuff manner, imo...
hth,
doug s.