Sub for MA Silver S1

Any suggestions for a subwoofer to be used with my older
Monitor Audio Silver s1's in a 2 channel system. This setup will be used in a larger 25 x 20 room. Thanks
Do you have a budget? Music, movies or both?

I believe that MA makes subs. One of their models should blend nicely.

I have a room about the size of yours and I run a little REL T3. It does nicely for music, but doesn't quite get it for movies. You might also want to check out the SVS subs. I've had my eye on a sealed model for music only.
What electronics will it be used with?

Does 2-channel imply music only?
SVS makes great subs. They have a 45 day return policy with free shipping.