Hmmm. Lets see
M-50 from 1982 120wpc into 8 ohm full range with decent thd and "clipping power" of 200wpc into 4 ohm, 1.1V sensitivity, .
M-70 same agewise, 200wpc full range with decent thd into 8 ohm and "clipping power" 250wpc into 4 ohm, 1.41V sensitivity
M-80 same agewise I'd guess, 250 wpc into 8 ohm, 330 wpc into 4 ohm both full range with decent thd, 1.55V sensitivity
M-800 from 1988, 170 wpc into 8 ohm, 230 wpc into 4 ohm both full range with decent thd (and even indicates dynamic power of 600 wpc into 1 ohm), 1.32V sensitivity
M-830 I didn't find
The M-80 looks the most interesting to me altho not particularly impressive any of them at 4 ohm but would think the sensitivity should be in range of the Yamaha avr you have but there have been some models with some issues in that regard, don't know where the 810 falls. I'd probably just get a Crown XLS 1502 myself.