Electronic cross-over on pre-amp out, to split signal to subwoofer and bookshelf speakers

The great thing about a unit like the Behringer mentioned above: it is an inexpensive way to test the concept with your system. And even if you later want to focus in on just what features you want in a less electronic device, you can resell the Behringer. 

Most folks I know either go with the SPL also mentioned above or with MiniDSP, which can give you Dirac at the same time.

Related to the Sublime... so you must pick frequencies in advance... here are kits for DIY: Collections – Xkitz Electronics

From the inventor of the RiPol subwoofer: Ridtahler: Produkte

I have a crossover to split surround sound left and right into midrange and tweeter built into a multiplexer from Ralph Stens. He also has both a kit, or he will build for you, a separate crossover: Active Crossover AFW1 - RStAudio.de

Like I said, recommend getting something easy to test with and to resell first, and then pick what you really want at what level of sophistication / electronics.

"If you were just looking to add more deep bass to you system, then go with something like what REL offers that lets your existing loudspeaker run full range."

I agree , I run my Studio Electric M4 full range with my REL's crossover set low and it integrates perfectly.   

If i were to go with an external X over it would be one of high quality so as not to mess up the high pass to the main speaker.   

If you like to crank it up and have small mains it's probably not a bad idea to implement a crossover but I would spend some $$$ on a JL, Marchand or SPL .  You want one where you can adj the high pass and low pass freq and slope. 

I went with the Dayton Audio DSP-408, 4 RCA inputs, 8 outputs. Extreme flexibility in adjustments; x over, mixer, delay, and eq. I opted for the Bluetooth dongle, which allows adjustment at my listening position via an app. Or adjustment via laptop and USB. The x over has adjustment for high pass and low pass points, slopes up to 24db, polarity, and more. Eventually, I substituted the wall wart for a LPS with substantial improvement. 

Relieving the tube amps and the midbass drivers of the bottom octave or so should reduce distortion on both.  Those large frequencies hog a lot of power from the amps and cause large excursions to the midbass drivers.  If you can find out your amps input impedance, you could try a couple of inline filters (I used 10nF silver mica caps at a cost of < $20)

80hz-ish is a good option for two reasons...it keeps the sub playing in the range where it isn't directional, and it leaves just about the entire vocal range to the midbass driver. 

If you don't already have a sub, you might consider using two subs to help break up room nodes.   Most subs already have a built in low and high pass crossover, which could make it really simple to try.

I have been intrigued about the Sublime products for a while. If I were to buy tomorrow, it’s what I would choose. Good to hear feedback from someone who owns it. I get you have to buy crossover cards, but they’re inexpensive, like $12, could be fun to experiment.  And you get to stay in the analog environment.