SETs tend to have problems playing bass due to how the output transformer is built.
Western Electric sorted out a way around that using a circuit known as 'parafeed' (parallel feed).
They are rare, as best I can make out because explaining how they work is tricky. But there is an excellent kit:
This amp can make full power at 20Hz; most SETs can't make full power even an octave above that. Distortion is lower too, since bass causes most SETs to make excess distortion (for them the simple solution is don't let bass into them in the first place). Since the parafeed amp doesn't present the power tube with any weird loads like regular SETs do, its distortion in the bass region is lower.
They can be just as musical as regular SETs. It should not be that hard to find someone who can built the kit for you if you can't do it yourself.