The f113s have a LP crossover but it can be bypassed.
I would yes (to an external crosser) just for this reason. Why send that low bass power to your speakers? That would keep a little extra power for the mid bass on up in the mains. The purpose of internal speaker crossovers is to separate the needed frequencies for the different drivers.
That being said I personally do not use an external crossover yet for my sub. Someday when the funds permit I would love to get one for the reason I stated.
Just my opinion
IMO, the main reason to low pass the mains is that they are (usually) out in the room away from the walls. This means that cancellation caused by reflected energy will cause seriously ragged response in the long wavelengths (low frequencies). If the mains are low passed and the subs are against the wall, all bass is generated close to a wall, low frequency cancellations are greatly reduced, and bass response will probably be MUCH smoother.
OTOH, if your mains are also against the wall and/or they produce little to no low frequency info and/or you are able to effectively EQ the low frequency issues, then the decision to low pass becomes less compelling. You still might choose to do so, depending on the specifics of your set-up, but it's not as straightforward a calculation.
Thanks guys,the fog has lifted somewhat,Kal your right as I use a Arcam AV-8 for movies and I use the arcam's X-over instead of the other JL's(4) which also go thru the 50a's.
For some reason I keep thinking I can't use an ext X-over with the 50a's,the subs have them(which two of them I bypass for movies,my AV-8 has the crossover for movies.You guys are right as I'd just use an ext x-over for the other two subs for the music end of it.
WTF,don't know why I couldn't see that before,to much gear perhaps,Thanks guys.
I agree with Marty's response completely. I'm using the F113 with my B&W 800's running full range. One other reason to do this is that by by-passing the internal crossover, the signal path has less connections and less to corrupt the mid's and high's going to your main speakers. My speakers are considered full-range and, therefore, I set the subwoofer crossover at around 50-55 hz. It integrates really well.