Well, Beltane's solo fiddle isn't going to punch through their wall of drums on its own - and frankly, without the (controlled) distortion, their sound would lose some of its impact. The only other side I noticed using amps - Hobos IIRC - kept them at a suitably low level to achieve a balanced sound overall. There may well have been others but being in a performing side myself, I didn't get to see them all.

And a very few sides had just one or two musicians, who couldn't be heard beyond the other end of the set - a bit of amplification might actually have helped there.