Need advice about purchasing a subwoofer

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I have been doing a lot of research on subwoofers and I keep coming back to a couple of things. First, my room size which is on the large size at about 18X15X12 and I sit about 10 ft from the "front stage". My house is attached to another (I live in San Francisco) and I want to be respectful to my neighbors as I plan on being there a while . Based on everything I've read, I should probably go with a 12" sub. However, given the details I've provided thus far, would something like a eD A2-300 be overkill since I don't need/want to shake the foundation? Second, a co-worker has offered to sell me her Velodyne CHT-10 with very, very "low mileage" for $130, I've seen it and it looks brand new (she bought it right before a divorce 6 yrs ago and used it for about 4 months and now she just wants it out of her house.) I know this is an older sub but the reviews from users have been very positive including this one from Home Theater Magazine:

http://www.hometheatermag.com/subwoofers/177/

I don't really know if subwoofer "technology" has advanced enough since this Velodyne was manufactured to not consider it - so given that, would the Velodyne be a good deal? And would it be an upgrade from my Sony SA-W2500?

Obviously, I don't know much about subwoofers but I know enough that I want to upgrade the Sony. I use my HT for 60% movies and 40% music.

Here's what the rest of my system consists of:

Yamaha RX-V663
Polk Monitor 60's
Polk Monitor 40's
Polk CS2

Thanks for reading this long post and would really like to hear what other might suggest/think.