My preference would be to split the subwoofer output from the RR1250. This will allow shorter cable runs to the mains without the two extra sets of connections (input/output per line) in the run. This will give better sound from the main speakers.

This will also allow you to roll off the low end response of the speaker to match the subwoofer using the bass management of the RR1250.

One additional benefit is that the internal amplifiers in the receiver will no longer be required to provide power in the bass region, allowing higher output at the higher frequencies delivered to the main speakers.

When I get home tonight I'm replacing my LFM1 + with a pair of LFM1-EX that are now sitting in my kitchen. I might just try the system with all three, although the LFM1 + will be leaving the main system (destination currently not determined). They will be fed from the single channel subwoofer output on the pre-pro.