Technically speaking there’s no reason why you can’t design a switched mode power supply to power anything, including a valve amp. The obvious reason switch mode has become popular is that despite being inherently more complex (and expensive) than a linear psu, it’s far less heavy. So, apart from sheer cussedness, why would you put a lightweight expensive power supply in a big heavy valve amp? I can imagine that in a practical design shielding might also be a problematic.