I still need to do the experiment where I put a second Revel behind the first facing to the rear and out of phase. Create a dipole so to speak.
Very interested to hear your impressions!
Here is what I'd suggest:
1. Connect them in the SAME polarity ("bipolar" rather than "dipolar")
2. Place them side-by-side with one facing forwards and the other backwards, rather than one behind the other; and
3. Try plugging the ports on the rear-facing speaker, and maybe even on both speakers. Balled-up socks are probably adequate; this isn't rocket surgery.
Here is my reasoning:
1. In a dipolar configuration I think you'll have too much bass cancellation. Try it and see.
2. When I've modelled bipolar speakers, the net frequency response gets worse the narrower and deeper the enclosure gets. This is why the Mirage M1 was wide and shallow.
3. Below the baffle-step frequency the rear woofer's output "wraps around", and at some point the wavelengths become long enough that the rear woofer's output starts arriving "in-phase-enough" to significantly reinforce the front woofer. So you may end up with too much bass energy if all the woofers are outputting bass at full power. You may end up with too much bass anyway, as the speaker was "voiced" without the expectation of any wrap-around energy.
(
here is an arguably relevant online article by yours truly; some ideas expressed therein are outdated.)
I've mentioned before that though I loved the Quads I grew dissatisfied with the lack of density and "punch" to their sound, and that even smaller stand mounted speakers I'd try seemed more dense and punchy. I got the Gradient dipole subs made specifically for the 63s (which made for a decidedly 2001-like monolith appearance). Interestingly though they were the best sonic match I've heard between a stat and woofer, the dipole subs also seemed to match somewhat the Quad's own "lack of punch" factor. They never did "slam" like the regular floor standing box speakers I replaced them with. This lack of punch vibe is something I always hear with dipoles (which in my experience has been almost always panel speakers, so I'm lacking experience no doubt).
Anything to this?
I used the Gradient "subs" too, and agree with your observations. More on that in a paragraph or two.
To my ears the tonal balance of the Quads was light in the bass and they had too much energy in the lower treble region, resulting in harshness on female vocals. Arrrgh! I tried a passive notch filter; added the Gradients to hopefully make the upper treble bumpage less audible; tried an active notch filter (switchable in the active crossover that came with the Gradients); paid someone in California fairly big bucks to upgrade them; added homemade wings; and finally broke down and bought the Cello Palette.
I never heard "slam" with the Gradient subs, but they were imo a nice improvement in tonal balance anyway.
I don't really know why dipoles don't have "slam". I speculate that it's mainly because dipoles inherently do not pressurize the room, and perhaps partially because the high electrical "Q" of a dipole-friendly woofer (and whatever its equivalent would be in an electrostatic or planar magnetic panel) arises from an unusually low motor strength, and ime there is a correlation between motor strength and "slam". If you ever eyeballed the magnets on those Gradient subs, they were pretty small. At best I think they'd hit like a pillow.