I have heard the 3A DeCapos with some tubes and SS power. In the same veign and nature, I compared the EPOS ES11 with the DeCapos, identically the same sort of driver set up, the midbass driver of the DeCapo has a stationary sort of phase plug, and the same goes with the EPOS midbass driver, in which just the cone moves. Both are crossoverless, okay, except for a cap between the midbass and the tweeters. With this sort of set up, the sound is exceptionally dynamic, with an amazingly fast bass that bellies the 20-5 watt rating of the Sharp amplifier. I have not compared it with a OpChip amp set up, but for whatever it is worth, the Sharp sound very decent and pristine in sound. When I added the First Sound Paramount HR MKII, it was sort of a overkill, but now, we had the signal of some tubes going through a digital amp section, which gave the entire thing a nice organic tone, perhaps not accurate, but still very nice.
With the Fostex horn set up, the amp provides incredible volume, but somehow, I think that horns and fullrange drivers prefer a different sort of amplification to sound full or best.
I concur with your experience, that perhaps there is more synergy between this amp and your JMR Twins.