I have owned a couple pairs of KEF’s and I really enjoy their sound. Given your choices for heavier music, I am questioning the move to another KEF speaker based purely on your amplifier. My feeling is you will be missing out, and probably already are, on quite a bit without also upgrading your amp to something with higher current capabilities.
Both speakers are 4 ohm impedance, playing metal and electronic through those with a 40 watt tube amp will sound okay, but you would be much better off with a high current solid state amp fronted by a tube preamp, assuming you really want tubes.

