I place greater value on temp comp over multi-stages.
AGMs do tolerate 13.8V. AGMs can be maintained with an automotive alternator or automotive portable charger(not a deep cycle charger). Some chargers do have an AGM setting.
I charge AGMs with a two stage MS SS10 PWM solar controller. It is temp compensated and I leave it running year round.
Stage 1 is the normal bulk when ISC Amps are passed through to the bank. Stage 2 holds Volts at 14.1 or 14.4 Volts, depending on jumper setting, and simply tapers Amps so low that the Turnigy reads zero. There is no float. In summer, I use the 14.1V jumper.
HTH;
John