Can I get better sound?

I'm pleased with the sound that I'm getting out of my JMLab Cobalt 806 / REL Q150e combination. I have the sub set up just as REL recommends ( wired directly into the amp spkr terminals along with my main spkrs). I only have 1 set of terminals on my amp. The 806's are rated down to 50hz +/- 3db. Would I gain anything by running all speakers through a crossover of some sort? By restricting any signals below 50hz to my main spkrs, will I gain any musicality through them? If a crossover would help, what kind would I use? Or, would adding a crossover take away from the music by simply being placed in the path????
Thanks for any thoughts.........
My experiences thus far is that using some sort of a crossover for your mains will likely degrade the overall sonic purity somewhat. It usually tends to sound better, at least with most kinds of not too dynamically demanding sources of music, if you run the speakers full range from your 2 channel rig, and let the sub just fill in the bottom by adjusting it. However, what I've also noticed is that you will get more dynamic range and capability from a system usually if you cross over your mains and let a sub do the bottom also! This is usually accomplished by using some sort of Digital pre/pro, like an AV preamp/receiver.
The downside there is usually that you're using a potentially sonically inferior digital preamp.(usually you can get better sound from hi end 2 channel analog pre's).
But, then it depends on your set up.
Ideally, I think it would be better,, to cover all kinds of rock and heavy dynamic stuff(movies), as well as instrumental/vocals and such, if you could use some simple filter for the main speakers, without adding any colorations or downsides to the sound. I have seen some simple passive crossover network filters that can be inserted into the way of the line level path to the amps for the main speakers. I'm not sure how pure those are at not affecting the sound, but I've seen them...not enough experience there unfortunately.
Regardless, with most hi-end 2 channel systems I've heard, you usually will be better off with most of what audiophiles are going to listen to, if you simply run full range to the mains, and fill the bottom with a sub in parallel. IF you know you're going to run heavy dynamic rock and roll with most passive speakers, you can usually get more dynamic headroom and capabilities by "crossing over" the mains to, say, 80hz or lower, and letting the sub do bellow that. You're effectively biamping the system, and taking the demanding strain off of the already(usuall) dynamically challenged passive main speakers. I have heard exceptions however, but they are few and far between in the audiophile world.
I've owned a lot of high end speakers over the years, and I like rock,techno, and heavy dynamic stuff(as well as movies), but also do classical, vocals, instrumentals,etc. And when I can cross over my mains to let the sub handle the difficult dubties, the dynamic potential jumps way up for the system. The mains don't strain, sound compressed, and the system just has an "effortlessness" to the sound(A HUGE RESON PEOPLE WITH PASSIVE SPEAKER DESINGS SHOULD BE CROSSING OVER EVEN THEIR LARGE SPEAKERS TO "SMALL" ON A DD/DTS PRE/PRO FOR MOVIES!!...THEY JUST CAN'T OTHERWISE HANDLE THE BASS WELL ENOUGH!).
If anyone out there has any other experience with some of the passive in-line filters out there, which can be inserted into the signal path to the amplifiers(main speakers), wich don't affect the sound so much, let me know. I would like to try some of them! Otherwise, I'm sticking to using a slightly less pure digital pre/pro to cross over my system for the more demanding stuff!
In my experience the REL's work best letting your main speakers run full range, with the REL filling in the bottom.

It is OK to leave some gap between the where the main speakers end on the low side, and where the REL takes over. You can then turn the volume higher on the subwoofer.
Many people setup the sub right up to the limit of the main speakers. This usually requires the volume on the sub to be set lower to reduce boom at the crossover/cutoff point, which negates some of the benefits of having the subwoofer.