Stacking 901s is not a line array in any shape or form. However, I wouldn't be worried about comb effect interference from stacked 901s. The majority of the sound is intentionally bounced off the rear wall making comb effects a non-factor.
Bose 901 Series ii
I own a set of Bose 901s with the equalizer. I have them paired to a Carver pm700 200wpc amp, and am using a Harmon Kardon 630 for the pre-amp and tuner? Any suggestions for something that might put out a little better sound and not break the bank? And yes I know, no highs not lows must be Bose.