Well our stuff works fine with the Westminsters! But any tube amp that can make 50-80 watts should work fine also. If I were you I would look for one that is zero feedback as it will sound smoother, and triode is nice for the same reason (less distortion).
At power levels like this, SETs have trouble making bass but the Westminster does not. Something to think about. Push-pull triode amps are out there but they are not particularly common. But the Westminster isn't either- IMO it is worth the search.
Now if you had about 6-10 db more efficiency then an SET would be a nice alternative. But I am of the opinion that if you **really** want to hear what an SET does, you really do need efficient speakers. Others do not agree, so I will characterize my position as 'conservative'. I don't like distortion as the ear treats it as coloration which is what I hear in a lot of SET installations (I don't hear that so much if the speaker is really efficient though). Most SETs will distort at the levels it sounds like you need with this speaker. So I recommend something that can do the power without strain.
Now another alternative would be an ultra-linear amplifier if you could find one that has a properly designed output transformer. But from what I have seen of most of them the ultra-linear taps on the transformer are not in the right place to really take advantage of the effect. This is likely because of the legacy of the ultra-linear patent; manufacturers got around the patent by misplacing the tap slightly, but they also lost the triode linearity that comes with the right tap position on the transformer's winding. This is why you read so much disparate comments about ultra-linear operation!
