I find tube amps that only have 8ohm taps usually sound a bit soft and rolled off in the highs with the 57's, as they are a 2.5ohm load already at 10khz and falling to 1ohm at 20khz.
This is why I prefer a good stable 30-40w Class-A solid state that can do the doubling, I built quite a few of these for 57 owners based on the Nelson Pass A40 and they loved them.
http://www.firstwatt.com/pdf/art_a40.pdf
This is why they say they best amp ever for the 57's, was the Mark Levinson ML2 25w Class-A monoblocks, that could double down to 1ohm.
https://img.ukaudiomart.com/uploads/large/2229206-mark-levinson-ml2.jpg
Cheers George