So my current setup is sort of like this - I run my sub using the speaker level inputs and don't run a XO on the LS50s. The sub has an LP XO, so I simply turn up the gain of the subs so that almost everything should go through the sub and then fix everything with REW. This means that in effect I get 20-80Hz handled by my sub. The sub has a built in rumble filter I believe, so if something hits that is below that, you'll see it come through on the LS50's in terms of woofer movement.i don't have the ls50's *but* i can't imagine leaving them run full range and adding a sub with the intention of high output listening... the entire idea of monitors is clarity , you lose that, full range at high volume with these...
It's far from ideal, in fact I would call it a bad plan and I don't recommend it. But it's sort of what I duct taped together given my amp didn't have sub outs. I plan to make things work properly at some point by adding a minidsp or maybe switching to AVR at some point.