The "Professional" amp is our reproduction of the 1955 5E5 "Pro-Amp" circuit, though ours is housed in a tweed "Deluxe" sized cabinet with a 12" speaker (instead of a 15").
The circuit is almost identical to that of the 5E3 (and our "Superbe") except it uses 6L6GT tubes instead of 6V6GTs and a 5U4G rectifier instead of the 5Y3GT.
This results in an output of just over 20 Watts and an increase in bass and treble (or a reduction in mids...?) It retains the low gain 12AY7 preamp which allows more (smooth) power tube distortion than (crunchy) preamp distortion.
While the amp has a generous amount of power supply "sag", the 5U4 rectifier provides much more current than the 5Y3 so the effect is much less than that of the "Superbe".