In general, tube amps have output transformers- OTL amps being the exception. So tube amps generally have high impedance output because of the transformers. That means they like high impedance speakers. When the speaker impedance goes low the amperage capability of a tube amp becomes limited. That means the tubes are not able to generate the same voltage levels at lower impedances that they are able to generate with a high impedance speaker. You can short the outputs of a tube amp and not cause any damage. The voltage goes to almost zero. (Don't try that with a SS amp- it will pop because current will try to go to infinity.) Tube amps typically have power output ratings that are the same for 8 ohms and 4 ohms impedances. In contrast, SS amps are low impedance designs. They typically increase their power ratings from 8 ohms to 4 ohms impedance. The big monsters with beefy power supplies can generate gobs of amperage and so their power ratings double from 8 ohms to 4 ohms and then to 2 ohms Some even double at 1 ohm. They probably couldn't run at that power level for very long, however, without overheating. Now a tube amp would have the advantage over SS with a 16 ohm speaker. The tube amp would maintain the same power rating as at 8 ohms- because it could generate higher voltages going into the 16 ohm speaker. The SS amp, however will halve its power rating from 8 ohms to 16 ohms- just as it can double going to 4 ohms, since it's amperage capability is reduced due to the 16 ohm impedance. SS amps and 4 ohm speakers were made for each other. Sure, some tube amps work too- ideally the output transformers are matched or have taps for 4 ohm impedance.
As for Class A power- it's simply a design standard based on the bias or offset voltage/current. I don't see how a class A watt can sound louder than a class A/B watt. Almost all SS amps and tube amps for that matter run class A to some power level and switch automatically over to A/B mode once the bias limit is reached. That power level is based on the bias setting which is related to the amount of heat sinks and idle temperature that the amp has.
As for Class A power- it's simply a design standard based on the bias or offset voltage/current. I don't see how a class A watt can sound louder than a class A/B watt. Almost all SS amps and tube amps for that matter run class A to some power level and switch automatically over to A/B mode once the bias limit is reached. That power level is based on the bias setting which is related to the amount of heat sinks and idle temperature that the amp has.