Subwoofer purchase

A sub that's properly placed and setup will blend with your mains and shouldn't be a distraction. A lot of people will tell you sealed subs are better for music but others will say a good sub is a good sub. What's more important is getting a sub that matches your room size. How big is your room?

A fantastic sub that blends great with Maggies is a Vandersteen sub, these have 3 8in woofers and its made for those who want music first, they will do HT all day long but its a music sub. Its the kind of sub folks get who dont want ghetto bass but fast tight and musical bass to enhance real music of all types.
While its more hit and miss I have seen HSU Research subs do a great job with speakers for music.
Martin Logan makes fast subs too that mate with their somewhat similar needs to a Planar speaker being its a hybrid electrostat.
I have heard some good results with Definitive Technology subs aswell.

Your budget is pretty tight for a great sub but I would look at Audiogon.com for used options, if you see something you like ask here or at their own free forums for experienced opinions.
Some love Velodynes, SVS, JL Audio but we are talking in some cases large dollars for these beasts.
If you or friends are handy you may want to look into DIY subwoofers as your money can go much further.

For music I'd recommend the Rythmik FV12. I have the F15 and it is wonderful for music. It blends very very well with my speakers.
I've also read that the Emotiva Ultra12 is one of the best subs in your price range for music.

http://www.rythmikaudio.com/FV12.html

http://emotiva.com/ultra_sub12.shtm

I have no connection with the seller but theres a Epik Sentinal for sale in the Classified Sec for $375.

The rumor or idea that 15'' subs are slow or not tight is false. The idea that smaller subs such as 10's or 12's are tighter or faster is also false. These are non factual rumors that unfortunately keep being repeated.

I have had all kinds of different sized subs throughout the years. In all kinds of different rooms and cars. The dual 18" subs I have at home now are the tightest subs yet. They simply dominant the air in my room, when called on. They hit and then they are done, they get out of the way.

Even the best subs can sound flabby if not set up well, pistonengine.

CS-18.T Duo. I have pics in the 2 Channel "let's see pics of your stereo" sticky thread and a review thread with pics in the AVS subwoofer forum.

Most people downfire them. They can front fire as well. The legs on mine have been removed and allen bolts put in their place to seal the cabinet. Put some vinyl bump-on feet on the sides and tip 'em over! I am waiting for metal grills to attach at a later date, I think that will look pretty cool.

I love Jazz and vocal Jazz, the REL T3 is a great subwoffer.