Try something: download a frequency test disc (my Devious Tiny Disc will work if you can't find anything better) and try playing it with and without the sub on. With the front speakers set to "large" you should get response from your front speakers (or their subs) from the start of the disc to the end, with the sub connected to the 950's sub output dormant the entire time. Then switch the fronts to small, pick a crossover point (80Hz would do fine), and try it again. In that arrangement, the 950's sub should output the signal from the start up to near 80Hz, when it will start to taper off and the front speaker pick up.
This is not correct: with all speakers "large", the sub should only get a signal when playing a 5.1 source (at which point it will get the .1 channel only).

One other question: have you calibrated the system since adding the sub connected to the 950? If not, you may have a level issue of some sort going on...