Or you could go cheap active, such as: Use the LF portion of a Behringer CX3400. About $100.00.
It will sum L&R out with variable low band x-o starting at 44 Hz. You could use the crossover in that to send a filtered signal to the satellite monitors, or you could go the blending route, and split your mon out to both the main amp and the CX3400. Done.
There's probably a lot of other options out there if you want to use your Hafler and not buy a plate amp (with built in x-o.) However, the Behringer is cheap and their recent build quality is good.
PS: Build up those 3 Pi kits and put them in your own studio.