M1HPAP — minimal specifications and with no stated reference level and conditions for anything listed, so you can't say much of anything here
Graham Slee Solo SRG / SM-PS — 120 ohms output impedance means it'll royally screw up (or at least alter; whether or not it's in a bad way is up to you to decide) the frequency response of many IEMs and some headphones. The output power levels into the higher impedances are lower than some portable amps, somewhat decently below relatively entry-level non-portable amps like Schiit Magni, FiiO E9, etc. Because of the high output impedance, into lower impedance headphones it's even lower. For many people it'll still be enough for most headphones, but some may want more with say 600 ohms Beyerdynamics and many of the popular planar magnetics models. The noise spec looks abnormally high for a headphone amplifier, regardless of whether it's in dBu or dBV (it doesn't specify, so maybe dBu); you probably don't want these for very sensitive headphones and definitely not IEMs anyway. Also, the gain is set rather high, meaning for sensitive sets you'd need to turn the volume down quite a bit. Only one distortion figure is given, and it's low, but only under an easy testing condition with high impedance load and not at max volume; it's possible this is an issue for blasting high volumes into lower impedance sets, but probably not.
Beyerdynamic A 20 — somehow you found another amp that by design has abnormally high output impedance (100 ohms). It can be a little louder than the above amp for high impedances but a little less at lower impedances. Still, it's more than you'd want for most headphones, but maybe not enough for some. Distortion again measured not in the toughest condition, but at least there it seems good. SNR doesn't give reference, so it's hard to say what was really tested and what it means. This amp also has kind of high gain.
For the latter two amps, I guess the high output impedance limits some of the potentially ear-damaging volume you could accidentally set for low-impedance headphones, what with the big gains. Anyway, as usual, all the listed specs are very much incomplete.
