Hello WhiteCamaroSS, I premise that I have not had the opportunity of listening to the Bel Canto REF600 yet
Bel Canto Ref600 is a basic implementation of the entry level NCore NC500 power conversion module and the unregulated NCore NC/1200/700 SMPS in a mono configuration.... What Bel Canto seems to have added to the basic module-in-a-box is their own signal imput stage to raise the balanced input impedance and reduce common mode noise.
This is a similar approach to the lovely Merrill Teranis stereo amp, which I evaluated a couple years ago.... As nice as Teranis was at its real-world $2500 price point, in my system it had a tendency of showing traces of congestion in loud/complex passages, and so fell well short of the excellent performance that I found in my system with the Merrill Veritas monos which are based instead on the full-blown NCore-1200 modules. In turn, Veritas performance is perceivably below the Rowland M925 monos that you and I are quite familiar with.
Furthermore, a couple of years ago, a friend of mine tested extensively in his own system REF600, Veritas, and the Rowland Continuum S2 integrated. He found Veritas to exceed comfortably the performance of REF600, but in the end he purchased the Rowland Continuum S2 integrated, because he preferred the amplification section of CS2 over Veritas. I am quite familiar with CS2, and I do like it very much.... However, it does not achieve the performance of M925.
My guess is that REF600 performance might fit somewhere between the Teranis stereo and the Veritas monos.... I am conjecturing that the Bel Canto might be a little closer to Teranis than to Veritas.
M535 is far from being a simple module-in-a-box amp implementation. Not that this really matters one way or another.... What matters is what the critter does for living... My extended experience with M535 in bridged mode, which I have had in my system running since last October and is fully broken in, and particularly after the application of a recently developed phase locking slip-on, is that its performance grazes M925 territory, an amp that both you and I love.
If I were to venture a performance order from lowest to highest based on the above, I would list amps in the following order.... Teranis, REF600, Veritas, Continuum S2 (amplification section), M535 bridged, M925.
So, while the above is admittedly somewhat indirect, it does never the less leave me scratching my head about the observations of your source on M535 bridged performance. BTW, Bridged M535 in the US are rare critters... While pairs have been playeing at audio shows, I know of only two extant pairs in the wild... One of them is in my own home. Has your source had significant experience with bridged M535?
Saluti, Guido