Bose 901 Series ii

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.

Stacking 901's was common in Korea. A hallacious soundstage was the result.  901's are not designed to be pure and accurate - they are designed for musicality and large soundstage.  89% of the sound is reproduced by 8 drivers in the rear at specific angles to reproduce a lively experience. 11% of the sound is directly radiated at the listener.  This is where the amazing soundstage comes from when 901's are placed right. Your HK pre is likely the most limiting component.  I'd sell it and get a good 2/4 channel pre/pro that would match the 901's.  You may consider selling the Carver amp and get a Sunfire amp with 300W or more per channel.  Also comparable would be used Marantz or Denon amps which would be good matches for the 901's.  Bose had a demo room in Hawaii with generic entry-level components and high-end components - the listeners could not differentiate in direct A/B comparisons.   

Oh, and rather than stack the 901's, I'd place one pair on the floor with the Bose 901 stands and hang the other pair from the ceiling.  (901's MUST be used with the Bose active equalizer - it would be near impossible to replicate the active eq as it perfectly creates an exact opposite frequency curve of the drivers which are mid-range efficient).

I had 901's as did many of my friends while I was in high school and early college.  The best match for the 901 speakers I heard then were Marantz preamp paired with Marantz power amp. Very "tube-like" and delicious mids.  The overall presentation was very, very good with a pair hanging and a pair on Bose pedestals.