I have a pair of the 970s from 2 years ago, similar to the 975s. The bass is just as good as a powered sub, it's thick and deep. Plus you actually have two subs because each speaker contains an amp and sub. Go to
www.bostonacoustics.com to see the differences between the speakers. I personally would not reccommend the 940s. They actually use a car speaker for the midrange. The two mids in the 950s and 975s sound much better to me. The 950s are the same as the 975s w/o the subs.
I use 970s for the front and 950s for the rear.
Main differences:
975 125 watt amp with 10 inch sub, 2 mids, 1 tweeter.
965 80 watt amp with 8 inch sub, 1 mid, 1 tweeter.
Of course, you must listen to them for yourself to decide.