Here's a short list of speakers to consider with your tube amp:
Audiokinesis
Classic Audio Reproductions
Coincident
Reference 3A
Devore
I believe the top two will be better on progressive rock because they have large bass drivers combined with horns, and they will move more air than the other speakers with smaller drivers.
This is not to say that the other speakers won't also sound great. It'll be a matter of personal preference.
FWIW, the Audiokinesis speakers are sold with a home trial period, so you can try them in your system and decide whether to keep them or return them.
Audiokinesis
Classic Audio Reproductions
Coincident
Reference 3A
Devore
I believe the top two will be better on progressive rock because they have large bass drivers combined with horns, and they will move more air than the other speakers with smaller drivers.
This is not to say that the other speakers won't also sound great. It'll be a matter of personal preference.
FWIW, the Audiokinesis speakers are sold with a home trial period, so you can try them in your system and decide whether to keep them or return them.

