I think you are going backwards. The M3si is a perfect match for the LS50s. The REL sub would seem to be a perfect match as well (using the high level input). The trick here is moving the sub around until you get optimal room pressurization (I'm not going to get into room acoustics and DSP here, other than to say attention to both can yield remarkable results). The other "trick" is having realistic expectations. The LS50s are only going to throw out so much sound. You could easily dial in too much bass with the sub. I have a similar set up (Harbeth P3esr speakers. REL t5i). I have both the crossover and level on the sub set at about 9 o'clock, just enough to add a bit of room pressurization. Any more and the integration with the Harbeths is gone.
If you are looking for big, thumping bass or home theater type sound effects, you can turn the sub up. But for most music listening, good integration is going to be a lot more pleasurable.
If you are looking for big, thumping bass or home theater type sound effects, you can turn the sub up. But for most music listening, good integration is going to be a lot more pleasurable.