Both the Rumble 25 and 40 are class AB and utilize SMPS (switch mode power supply) that is more typically used with class D amplifiers. The difference in weight is that the Rumble 25's cabinet is constructed of heavier MDF while the Rumble 40's cabinet is constructed of lightweight plywood. Rumbles 100: 200; and 500 are all class D."The 100 is 22lbs, barely more than the 40 and not much larger"
That's a great selling point. In my case the $100.00 upcharge was why I picked the 40. Traditionally 60 cent a watt is a good price. A very effecent amp too, but I'd have had to go for a heavier battery. I would have picked the 25 but it's not a class D amp and heavier than the 40.
Fender Rumble Power Amp Info
You'll keep it. It'll be far too cute to send back.