You could buy a preamp and use the poweramp in your 5150.
A/B1 > 5150 > Send > A/B2 > Return.
First A/B switch would be your output (preamp) selector, then the 2nd A/B switch would be your input (preamp) selector.
Basically you would need to press 2 footswitches to align the input with the output - Or I presume you can get a splitter between send and return that could use both of the preamps as an input at the same time, while only 1 will ever actually be active at any one time - In which case you could use only 1 A/B switch to choose between preamps.