OK, I have pretty much ground to a halt re: comparing M3s with Sierra-1s. The M3s are quite a bit more forward than the Sierra-1s (most reviewers describe the Sierra-1s as "warm" which AFAICS is the opposite of "forward") and it's really hard to tell if the other differences I hear are a function of frequency response or something else.
So I had one of those "Mein Fuhrer, I can walk !!" moments yesterday when I remembered that the receiver I was using had tone controls, although I had not touched them since... forever.
I turned the treble down just the tiniest smidge, maybe 1 dB (as small as I could do without the detent pushing it back to zero) and that shifted the balance maybe half way towards what I heard from the Sierras and seems to give just the sound I want.
Ian reminded me that I had already encountered a case where the optimum room treatment changed a bit depending on the speaker I was using (believe that was with the M60s and replacing the couch) so I'm going to revisit the blind from a few posts ago (the one that made Sierras sound dead & lifeless) and see if I can get similar results without tone controls when time permits.