Hey!

I'm kind of unfamiliar with live set ups but basically what I'm trying to do is add some low end to my Behringer 210D PA set up. I have a nice old Acoustic Bass cabinet (big old thing) that I'd like to hear the low end through. I make dance/house music.

I don't have a crossover unit.

Before what I did was route the bass sounds directly from my MPC through the head/amp to the bass cab.

Now I have a different setup and want to send multiple low frequency instrument channels (bass synth, drum machine) to the bass unit while sending the mid-high end channels to the PA.

Is there a way to do this through a mixer? I have this Mackie CR 1604 Mixer...

https://ibb.co/bHpkHRR

I've read that a thing to do is create a 'submix' out from the mixer to send to the bass speaker/subwoofer. But isn't this essentially like adding a bassy center speaker (i.e. copying the master input and sending it to another speaker?). I'm looking for good stereo separation and I'm think this might muddy that. What think you?

Maybe it's best to mix the low end instruments on a separate small mixer and then route them to the head/sub?

Is using a crossover the way that most people achieve this kind of thing? Thanks!

Sounds like a setup called Aux fed subs would work for you.
Google it.

If you just want more low end in the system you can just feed a line signal to the bass amp full range. If you only want certain things going to the bass amp only then use a splitter or use an aux signal.

if not used to feed a recorder or an additional speaker system, you might use the mackie's mono output to feed the bass cabinet full range - possibly the most simple way and doesn't take away any buses/auxes.

now how suitable the bass cabinet will be is a different question, the (possibly) asymmetrical hi-z input another potential issue, positioning yet another topic...

As a bass player, I feel obliged to point out that it's a bass cabinet, not a sub-bass cabinet. It probably only goes down to 50-70Hz.

I think the OP knows this. He just wants more sound and fuller bass.