This story may or may not be relevant to you, but since it involves an actual experience of going dual mono with Devialet, I'll inflict it on you anyway. It's complex and there are some twists and turns ...
Several years ago I was using a D200 to drive Martin Logan Montis. I became disillusioned with the rather anaemic balance of the system. So I set up a comparison: I borrowed a slave D200 from my local dealer, so as to run the Devialets dual mono, and I compared the D400 dual mono config with a Sanders Magtech power amp. The dual mono config didn't seem to drive the MLs any better than the D200. However, the Magtech turned the MLs into the wonderfully full, rich holographic speakers that they should be.
I say this story isn't relevant to you, because there is a known issue with Devialets driving electrostatic speakers that have vanishingly low impedance at high frequencies. This won't apply to your speakers. Even so, I did find it interesting that going dual mono made no difference.
I should also say that I still have the D200, and it's now driving a pair of JBL 4429s, to wonderful effect. I rate the D200 + JBL combo very highly.
Recently, in a weird mid-life crisis, I bought a second pair of the JBL 4429s. So I now have two systems with same JBL speakers. One pair is driven by the D200, the other by a 400WPC Hypex power amp (from Nord Acoustics). TBH I don't think I can tell any difference between the two amps. They're both outstanding. In other words, I'm not sure the extra watts of the Hypex amp make any difference, and I certainly wouldn't think of spending my readies on turning the D200 into a D400.