@tablejockey it's not just a rectifier tube swap, it's like having a separate power supply. The B+ is supplied by the super rectifier and the equipment you use it with supplies the filament voltage for the signal tubes.
Space Tech Lab super rectifier
I had a chance to listen to this and was extremely impressed for those who also may be curious like I was. I'm not sure how these devices can do this but I can certainly hear the differences, it's not subtle.
Happy listening,
---daniel