Anyone With A Class 'D' Amp Play A 4 or 2 Ohm Speaker?

Sounds to me like despite all that time, you've still not unlocked all the performance available.

What you're not taking into account is how variable ESLs are! Old Quads are higher impedance as are Sound Lab and certain Acoustats. Other Acoustats are low impedance and Martin Logan a bit lower.

My point is no one single solution is correct and the math bears it out. FWIW, about 80% of our MA-2 production is on Sound Labs and I too have been working with ESLs since the 1970s.

Please don't put it down to one way and one way only. Such generalizations are simply false.

The OP has Infinity Kappa 9’s who’s impedance drops to

Bass
.8 ohm at 35hz
1 ohm at 40hz
2 ohm at 55hz
2.8 ohm at 60hz.

Upper mids lower highs
2.5 ohm at 2.5khz
1.2 ohm at 8.5khz.

All this and we haven’t even taken into account any negative phase angles yet, which will make these figures even more severe, pity there’s no graph on this.
.http://www.classicspeakerpages.net/IP.Board/uploads/monthly_2016_03/Kappa9Impedance.jpg.340a528e36b90ff2b94f383547346fb2.jpg

This is clearly a speaker that need bags of current for them to work at their best!!

https://forum.audiogon.com/discussions/help-power-pair-of-infinity-kappa-9-s-please

Cheers George 

Theta Prometheus monoblocks powering Wilson Maxx 3 speakers. Class D and no issues driving them.

Ralph, thank you for your excellent explanations, as always. My Martin Logans definitely sound more balanced in the top octaves when driven by my McIntosh MC2200 as compared to my other SS amp that doubles down with halving of the load impedance. I'm speculating that the autoformer design in many of the McIntosh amplifiers is why the Martin Logan pairing with McIntosh gear is very popular. I wonder if a pair of MC601s could be a good match for the OP's speakers.