Hi I have also SP200 and SP400, but different DACs (SMLS SU-9, Topping D90 and Topping D50s). Overall the SP400 is definitely cleaner and more calm, SP200 is always slightly more nervous in direct comparison. No problem with power output with my AKG 701 headphones on both. With balanced input, I have no need to change from low gain and volume on SP400 is between 40 and 80.
SP200 is little more warm sounding, but I still prefer SP400 without question. Vocals and high octaves are much cleaner.
My favorite DAC is currently SU-9, unfortunately I'm not familiar with M400.
If you don't need remote control, you can also consider Topping A90. In direct comparison it's warmer sounding than both SP200 and SP400 with more robust bass. The warmer balance makes it sound not as super transparent as SP400, but details are definitely here, only slightly tamed down.
Both SP400 and A90 are worth the upgrade, I can live with both of them. I'm going to sell SP200.
According Sandu's review you can consider also SMSL SH-9, if you don't need pre-out.
https://soundnews.net/amplifiers/headphone-amps/smsl-sh-9-thx-888-headphone-amp-review/