nothing official. more preferences. like equal dacs, but with a different chip maker for each.
i know peeps that bi-amp and tri-amp, and use d for the low end and class a or a/b for highs and, depending, mids.
theres lots of debates on other forums that are more DiY and speaker oriented. do a search for class a vs class d for speakers.
class a for sure gets hot. class a tubes has less issues. the tubes dissipate a lot of the heat. while not an official way to calculate, a rough estimate is to take efficiency and double wattage for each db your trying to get. i.e 87db at 1 watt, 88 db at 2 watts, 89db at 4 watts, 90db at 8 watts, ect. take into account standard measuring practice is 3feet/1m away.
and personally, i will almost always recommend upgrading speakers before the amp. there arent many bad amps anymore. and system value tends to go with speakers over amps.