I don't have any experience with either the Abyss or the Dynamo, but assuming they are comparable in quality (not quantity), two smaller subs will usually integrate better with dipoles than a single larger sub.
You see, dipoles inherently have smoother in-room response than monopoles do.
Among monopoles, in general the more low-frequency sources you have spread around the room, the smoother the net in-room frequency response. With only a single sub, there's a pretty big discrepancy between what's happening in the upper bass and what's happening in the deep bass. Two small subs are better than one big one in this regard. With four properly set-up small monopole subs, you can get a very good blend with a pair of dipoles. That's probably the point of diminishing returns.
One other thing - make sure that the crossover of your sub(s) rolls off steeply, like at 24 dB per octave. In my experience, it is harder to get a gentle slope low-pass filter to blend well with main speakers that have a fairly steep roll-off (Maggies roll off at 18 dB per octave on the bottom end). Of course, Richard Vandersteen would disagree with me.
Best of luck to you,
Duke
