use one of the jl's mentioned above if you are worried about long rca runs. they will do the same thing but use rj45's for the run to raise/lower the gains. a bit more money than the lc-1's, but you might save the money in not having to buy good quality rca's for the long runs..
otherwise yes, you can use the lc-1 before the splitter.
Yep, you've got it, lc1 befor the splitter.
FYI you can surface mount or thru dash mount as well.
On the other hand, I do like the idea of restoring the input voltage to the JL M6600 since I've got it split in half right now as well as the elimination of the long RCA runs. This might be worth the extra $40 or so for the JL Audio CL-RLC line amplifiers over the LC-1.
unless you have spare rca's laying around, a good set of them are going to run $10-15 each, so the true cost is much less.
One word of caution when discarding the case and flush-mounting the LC-1. Because the threaded shaft is so short you may have to eliminate the case. In doing so you've lost the strain-relief for the RCA cables where they're soldered to the board. As a result you'll lose these on the first choppy day. So add a small splint and a little silicon and heat shrink over the circuit board, RCA, splint and silicon until the silicon oozes out a bit. Loosly strain relief your RCAs with zip ties behind the LC-1 and this should last forever.
Note, the JL control has a longer shaft so it wouldn't be necessary to break it out of its enclosure.