Good suggestions from everyone who has chimed in. I'd first check to make sure the center channel is okay. Use it by itself and see how it sounds. If it sounds okay, get an analog SPL meter (Radio Shack) and make sure the output dB is the same as the front L/R. If that's okay, then it must be something to do with the receiver, perhaps the mode you have it in. The lack of surrounds shouldn't make any difference at this point.
BTW, I'm not aware of any Stratus series surrounds. The need to match the surrounds is much less crucial than matching the front 3 anyway.