FYI, the Doyle book lists the Tremolo options of the JTM45 head as: model T1987 and T1989
With the 1989 being marketed for organs. The catalog picture with the models is on page 165 of the first edition of Doyle's book.
Btw, you don't necessarily have to get the 12W Mills. they are pretty big. But the 12w and 10w are good for 1000Vrms. If you aren't familiar with Mills, they are what lots of manufacturers and hobbyists use for high quality speaker crossovers.