Subwoofer other than Strata 3

Rel Storm III can be found used w/i your budget; although, it's a ported design, and I assume your reference to non-ported speakers implies you want a sub that's also non-ported?

Attilasimon - Try and find a good deal on a Velodyne DD10. Very musical sub, much more flexible than the REL

Look at MJ acoustics, similar in design to REL (some say, the same people)

http://www.mjacoustics.co.uk/

Check out Vandersteen 2Wq subs. Probably the best for integrating with your main speakers. They are unique in that they connect to your amps speaker terminals and this in part is why they integrate so well. From the Vandy web site:

Conventional powered subwoofers receive their input signal directly from their crossover before the main amplifier. The sonic signature of the main amplifier that is an important part of the sound you hear from the full-range speakers is missing from the subwoofer. This causes blending and integration problems as the sonic characteristics of the system are different above and below the subwoofer crossover point. This deterioration of system coherence is why conventional subwoofers have never been totally accepted as part of ultra high-performance audio systems.

The 2Wq uses a unique, innovative connection method that reduces the current demands on the main amplifier, but leaves the main amplifier in the signal path to the subwoofer. The system realizes the benefits of bi-amplification with absolute sonic continuity as the main amplifier’s characteristics that are evident through the full-range speakers are maintained to the deepest bass, but with the power and control of the 2Wq ’s internal 300 watt amplifier.

Complete description: http://www.vandersteen.com/pages/2Wqlit.html

Thanks for the input
Storm 3 being ported I feel would be a little boomy.
The DD10 I can try from tweeter-I did hear they were better for movies than 2 channel, however I could be wrong.
MJ-not sure-I'll do some research.
The 2WQ's do come up used and I have heard they are great
for 2 channel.Not to spark the 1 sub verses 2 sub debate,
but the 2WQ's seem to be usually sold as pairs.Do they work well as a single sub? My room is rather small at 11.5x17 so
2 subs would be tough.That is why I wanted the Strata 3-single sub ok,non-ported unit,small footprint,reasonable price
price.
Thanks,
Attila