Possible to use sub without preamp?

You can use a sub that has a speaker level hook up like the REL that is designed to sound best that way. You would run them in parallel without any degradation. The REL will actually emulate the sound of your amp to make it seamless.

You can use Monster solid brass 1 in 2 out splitter and as long as you use a left and right one to drive the sub in stereo, your output on your digital source should be able to drive that load with no issue.

Both of these options are better sounding than most outboard crossovers.

Hevac1 is correct as well but most high performance subs don't have speaker hookups except for REL which actually comes with a hook up cable that hooks to the back of your amp.