I'm pretty sure your sub's high level inputs are used to drive the internal power amp. This is the best way to use a sub, with high level inputs, because your sub and speakers are receiving the same exact signal. It is said that the PRAT is better this way...
You have to try and see. The receiver will send voltage only to the sub which will have a high input impedance, so it won't effect the sound of the BOSE sats.
The grit of the SB digital attentuator is not audible until you get down around 70 on a very high resolution system. 1 bit is thrown away for each 6dB of attenuation. If you are using mp3s, don't even worry about it. You can play a wav file with the Rx turned up loud and the SB turned down to 10 and compare to the reverse. Use a track with pretty tones, although even thrash will sound like crap at 10 when signal sent to DAC is reduced to 8 bits (telephone quality). Listen for the grit and blurred treble.
Also experiment with just sending one channel to the sub instead of both. On pop/rock the bass channels are usually almost identical, because bass instruments are almost always put in the center. The advantage can be seen when you count how many drivers the sub has, compared to the sats.
Bass will be noticeably blurry when you sum the left and right signals in a track which has any appreciable stereo separation at low freqs, like jazz and classical, and some rock too. With the Bose sats you will probably want to play the sub up to a kinda highish crossover point, where you will find more and more stereo separation as you go up, and where your ears become more and more sensitive to blur and location. If the bass is located between the sats, you will not notice only one channel bass. Worth a try.
Wife's out of town, so I can play as loud as I want
Party at Nicks!!!!