hedgepig, if you look in any home cinema magazine you will know that they have all raved about the REL Quake.
If you ask on here, most will tell you it's really a crappy boom-box and nothing but a mid-woofer.
I took the opportunity to get a demo of this recently (not because I wanted to buy it, but because of all the controversy over it). FranklyI wasn't impressed, to my ears all I could hear it doing was "colour" the sound. You should heed Uncle Erics advice on this one (omg did I just say that?
).
The subject of subwoofers is one of the most subjective discussions, and "which is best?" splits this forum right down the middle.
I would strongly recommend you look for 2nd hand REL subs, I particularly rate the ST series, having owned previously both the Strata and Storm models. I also used a Q-Bass 50 and until I sold it, it served me well for several years. It really is a class piece of kit (though try and get 2 if you can
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Don't look on ebay, that's where I sell them
[they go for silly prices]. Keep your eyes peeled on the classifieds section of this forum, you will get the best prices here, and if in doubt, ask in this section what things are worth.
Failing buying a 2nd hand REL, you could also look at the powerbuy section, closest to the price range of the Quake is the Velodyne CHT-10 which by all accounts (I say this as I've personally not heard it) is unbeatable in this price bracket.
Size may be a consideration, which is why perhaps you're thinking about the Quake (?), the subs which I've mentioned above are ALOT bigger than the Quake mid-woofer.
There are other, smaller subs out there, but someone else would probably need to comment on them. As Eric says, with your size room though, with a small sub you're likely going to need 2 of them which will take you into the price-bracket of some really "kick ass" subs (but bigger ones) - think Paradigm Servo 15 and the Velodyne CHT-15 (£500 and £685 respectively).
One thing you should do however, regardless of your priorities/budget/circumstances......is go and listen to some side by side. There really is no substitute for hearing kit with your own ears. Also if buying from a high street retailer, make sure you have the option to return the item if not suitable. Subwoofers, just like any other speakers/kit etc......will likely sound v different in your own room as opposed to a dealers demo room.
Regards
Matt