I have owned Quads, ML, and Magnepan. Specifically, Quad 57’s, 2805’s, and 2905’s; ML Prodigy; Magnepan Tympani 1A, IIa, IIIa, SMG. Have just ordered Mag DWM and MMG-W for the HT.
Considering that you already own Sound Labs, I would not consider anything other than Quads. ML hybrids are OK for HT, but I found them disappointing for audio.
Quad 57's are a very musical speaker, but lack the bottom and top octaves of the new series, and also their clarity. That said, the best room I ever heard used 3 pairs of 57's, stacked.
The 2805 is much brighter than the 2905, which is quite mellow for an ESL, and the same size as your M545. Both can be significantly improved by a toroidal step up transformer and a better HV power supply, which I suspect brings it near the performance you already have.
Good luck - I’ve always wanted to try SL, but never got around to it. Maybe someday ...