I've heard the whole Boston lineup at my local shop. The SX60's sound good, but for $145 you can get some SL60's from
ebay, which is a step up, and in my opinion, a better sounding set. With the SL60's you have seperate passive units for tweet and mids.
SL60's
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=9742630745
SX60's
http://cgi.ebay.com/BOSTON-ACOUSTICS-SX60-6-5-COMPONENT-SPEAKERS-NEW-PAIR_W0QQitemZ9744455067QQihZ008QQcategoryZ32819QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I would go with the SL60's as they are a step up from the SX line, and they have a really nice warm sound. I'm sure others on this forum could recommend some other reputable brands for around the $130 or less that would sound great and be perfect for rear fill like MB Quarts, Infinitys, etc. I liked the SL's so much, ima put them in my front stage and move my CDT CL-61's to the back.
As a company, Boston has always been up there when it comes to quality and performance. Their Pro and Z lines are considered excellent sets, and hold their own compared to even more exotic brands like high level Rainbows, Seas, Oz, Dynaudio, etc. I would really only consider these however if your wanting to brush up on your frontstage.
In a nutshell, your prob not gona wanna spend too much on rear fill, but Boston is a great company with years of experience in the industry providing excellent products. Even their more budget sets mentioned above should not disapoint.