Understandwhat you are saying tamz, but fitting a spirotech magnabooster in particular on the flow would be near enough useless. By design they catch most of the crap by way of a vortex system, so must be fitted the way it was designed- On the return. I dare say other products such as the TF1 by design might work on the flow to a degree but as far as I am concerned it would be pointless and unprofessional to knowingly fit this in contradiction to the instructionsAlthough they are primarily there to protect the boiler, better one fitted somewhere than not at all. They don't catch all the carp in one pass anyway so whether it is fitted to the return or not, if there is crud in the system, it will go through the boiler.
Still it should be on the return to give it more of a chance.