Steve,a higher rail voltage on the Stratos mono amplifier compared to Stratos stereo amp would give you the difference. The power transformers VA rating doesn't have to change when the output voltage from the secondary windings is higher, this is just a different turns ratio between the primary and the secondary on the same 400VA core.
Re the output devices being the same across the line. Its more cost effective to buy all the same output devices, in this case ones that will support 180 RMS watts/ch/8ohms. This gives you a more reliable amplifier in the field and a quantity discount of the total number of output devices. You have chosen the best sounding output device for the amplifiers you want to build and you can offer amplifiers with different power outputs to meet the customers demands without trying to find more instances where the devices with the correct spec for your circuit also happen to coincide with a good sounding transistor.
I am making an assumption in regards to Klaus's iterative process here.
Scotty
