John, as previous replies have indicated, the "sub-out" on your 3270 is merely a duplicate pair of stereo pre-outs and has no crossover or any other type of bass management on it. Therefore you have to rely on the provisions of your 6i for bass management. Although you speak at one point of taking its crossover "out of the loop", you can't do that unless you want the sub to run full range up to 200Hz or so. Since the 6i has a 12dB/octave 80Hz highpass on the line level outputs, this should be ideal for both the M2s and M60s(no change is possible even if you wanted it, it's a fixed 80Hz)and allows the 6i to handle what it does better than even the M60 with its good low bass. As far as the crossover control on the 6i, it should be set identically for both the M2 and M60 to match the 80Hz highpass crossover. Note that this might not mean a 80Hz setting on the control, since such settings on subs are notoriously inaccurate(although Nousaine's 6i test which I previously linked for you implies that the crossover control is more accurate than most)and might call for experimentation in the 60-100Hz area. The sub level setting might be slightly lower for the M60s since their rolled off response below 80Hz will be stronger than that of the M2s.
As for comparisons, sure the M2 plus sub vs M60 with or without sub which you mentioned would be interesting.
