@RapaNui: I'm not sure what the symptoms of too much gain would be, but I can report that the volume control in my setup behaves relatively similarly to the volume control on the integrated. I haven't measured this precisely, but subjectively I achieve the same volume level with Moreplay at 25% as I do with the internal integrated pre-amp at maybe 35% (?) rotation. I certainly feel as if I have plenty of nuanced control.
I should also admit that I'm making the assumption that the Home Theater bypass bypasses the M6si pre-amp section completely, although I'm not positive of that. It certainly bypasses the volume control. In my office, I have an old NAD integrated amp with a pre-out and main (power-amp)-in and removable jumper between them. I'm going to try Moreplay with that amp too.
I should also admit that these are my first forays into tube amplification, so I don't even know whether connecting tube pre-amps to solid state integrated amps is something many people do. Seems to be working really well for me.