Em, no your 3390 is a 2-channel receiver without the bass management provisions that multi-channel Home Theater receivers have. So, your M3s continue to run full-range and the sub doesn't take any of the low bass load off of them. The control on the back of the sub that you're referring to is just a low-pass filter that rolls the sub off above the frequency set; it doesn't affect the speakers. The idea is to try to set it at a frequency, maybe 60Hz or so, that blends with where the M3s roll off naturally. Making this tougher is the fact that the frequency markings on the control often aren't accurate.
Incidentally, a separate DAC isn't needed, since any equipment that involves a change from digital to analog(e.g., a player or receiver)already has a DAC to perform that function.
