Dazz,
Your findings do not surprise me. I would expect the BC amps to have no problem handling the natural impedance curve of the Merlins. No additional tweaking of the impedance matching between amp and speakers should be needed.
Your findings correlate to what I have found using BC ref1000m amps with various speakers of similar design. The BCs lean towards the leaner side of things when presented a fairly benign impedance curve but bring out the best in speakers that are more of a challenge by design, which is the case with many popular smaller designs that also have good low end extension capabilities these days, like Merlin. You just have to throw the kitchen sink at them power and current-wise to enable them to do their thing to the max.
The efficiency of Class D in general allows that to happen these days in a much more compact and hopefully affordable package than is possible otherwise.