why will the push-pull set have much more power, both would have two 6BM8s ?
Mainly, because of three reasons:
1) you can get double the voltage swing, effectively, with two tubes in PP, on the same power supply voltage
2) Even more importantly, you can run the tubes in Class AB1 mode, instead of Class A... this REALLY reduces the idle power dissipation in the tubes, which makes for more "left over" for audio output power.
3) the transformers can be MUCH SMALLER for the same output power in PP, as compared to SE, since they do NOT have to handle DC. This reduces the tendency to saturate... which makes for more power for the same weight of iron. Hence, it's much cheaper to make a 10 watt PP output transformer, than a 10w SE output transformer! This doesn't make any difference really, with regards to the same tubes... but it DOES mean that MOST manufacturers can make units with far more power, for the same price, using PP. This is a DISTINCT motivation for manufacturers!! In addition, not having to have a DC-saturation-blocking air gap in the laminations of the transformers can ALSO increase the magnetic efficiency of the transformer, which WOULD result in more effective power for the same tubes.