I agree on the power issue, most speakers are damaged by an underpowered amp driven into clipping and sending too much energy to the tweeter (distortion products) or exhibiting instability at clipping. Woofers are more robust and will handle overloads for a longer period of time due to the ability to sink more heat/stronger construction. Tweeters, on the other hand, can go silent with deceptive ease...
With regards to the sub issue, your requirements in how much clean bass you need will depend on environmental concerns (sturdiness of construction, ground floor vs. upper floor, nervous neighbors) and your own wants in terms of level. As far as frequency response, flat 20Hz ability won't be needed as room gain will still give you plenty of bass even with a sub that rolls off at 30 or 40Hz. Listen for quality bass before quantity, with one-note thudding wonders shown the door. In general, 10-12" subs seem a good suggestion- shun the $399 15" sub with "400 watts".
Other issues are placement and future plans. A physically large sub may preclude proper placement, and believe me with subs that's job #1. Even the best subwoofer poorly placed will boom and thud, which is fun for about 15 minutes until you wonder why everything sounds the same-poorly defined and booooring. Unfortunately, smaller rooms (and particularly rooms with similar dimensions or multiples thereof to their height/width/length) are particularly nasty in their resonant behavior. Use the reciprocity concept (woofer @ seating location, you waltz around the room listening to only the woofer playing, ears @ seated ear level, guaranteed to make anyone watching positive you're around the bend http://www.avsforum.com/ubb/smile.gif
), and start at or near a corner (front best) of the room. It makes a BIG difference. Once you've located several good spots, play the sub along with the other speakers (after you've put yourself back in the listening location, of course) to make sure the blend is right. Use pink noise or a repetitive bass line, whichever is more annoying. As far as future plans, if you're going to move fairly soon, perhaps you'll want to buy more sub than you'll need at present. Or, be like a friend of mine and simply add more subs as you go. He's up to six now.
[This message has been edited by Aerlith (edited July 27, 2000).]