Sub suggestion for Monitor Audio Radius surrounds

I currently have the components listed below and I am looking for a good sub-woofer and rear surround speakers along the same budget to complete a 7.1 system in a rectangular living/dining room that is approximately 10'x22'. I've read that the front L/R and center should be from the same line. Now I need a proper sub and rear surrounds that will be compatible with the above speakers thgat were purchased in approximately 2005. Is it essential that the sub and rear surrounds be from the exact manufacturer and line to blend fairly flawlessly?
My use is about 70/30 music (CD, LP, ipod) and HD TV (DVD Blu-Ray).

Panasonic 50PZ800U plasma TV (couch is approximately 12' from TV)
Onkyo TX-NR809 receiver
Oppo BDP-93 blue ray player
Direct TV HDDVR
Monitor Audio Radius 270s (front L/R)
Monitor Audio Radius 180 (center)
Monitor Audio Radius 90s on Monitor Audio stands (L/R)
*to be added
- power conditioner
- universal remote
- looking into Wadia DAC for ipod


Thanks!!

Neil

Thanks. I have to admit I don't know a lot about subs having never owned one nor set up a 7.1 surround system. I am learning quite a bit on this site and I appreciate the assistance from other members.

I'm not sure how much to spend on a sub, but I want it to match well and not be way out of league with the other components. I was thinking about the Emotiva X-Ref 10" ($499) after reading good reviews on their earlier sub line.

Thanks