@james633 --
Finding the right way to implement your subs will be a trial and error process. Your JTR subs have a DSP section built in already, and your JBL 4367’s are passively configured. Have you tried running your mains full-range and then just blend in the JTR’s via their DSP with a bit of overlap to where JBL’s start rolling off? Yes, I know you won’t take advantage of high-passing the JBL’s, but introducing a less-than-stellar DSP on top of your passive JBL’s isn’t ideal either, as you’ve found out, and so I would simply start out by leaving your mains alone and concentrate on moving those JTR’s around to where they integrate the best with the smoothest response, and top it off with delay and volume adjustments. I’d be inclined to position the JTR’s as close to the JBL’s as possible, but it mayn’t be the best way to go about it all things considered.
If you want to consider high-passing the JBL’s you need a quality DSP. I use a Xilica DSP/digital crossover (now re-branded ACX, but other than that they’re the same product and built the same place) which is an excellent pro line product and fairly priced. Yes, it involves A/D to D/A conversion steps as there’s not digital input, but don’t worry about that. Trust me, it’s a great DSP and sonically transparent. The Xilica/ACX DSP’s offer a vast array of filter settings far beyond those found in most all-in-one subs, that only have the most rudimentary dittos.
If you want to go a substantial step further I’d replace your 4367’s with their pro oriented siblings, the JBL M2’s in active configuration via the recommend Crown iTech amps with built-in DSP. I’m assuming the Crown DSP will be able to high-pass the M2’s, and then add a pair of JBL SUB18’s with an additional external iTech amp and be done with it. I’m not familiar with the DSP section of those Crown amps, but I take it they offer fairly elaborate settings that will eclipse those offered by the JTR DSP.
With the combo of the M2’s, SUB18’s and a trio of Crown iTech’s you’d have yourself a fully active setup (i.e.: sans interfering passive crossovers) with quality outboard amps and DSP’s that would take the sound of your setup to the next level. You’d be able to juggle with crossover frequencies, delay etc. to best accommodate your needs in ways you couldn’t now, and not least looking at each speaker channel of main + sub as an integral part and not a pre-existing main speaker with passive crossovers where the sub of a different brand and driver type needs to be "latched" on with a different, built-in amp and so-so DSP. Don’t underestimate the importance of using the same amps from top to bottom - including the subs, which most never get to experience since most subs house built-in amps.
Yes, the latter option will be the most expensive route, but hey, sell your existing McIntosh amp, 4367’s, K235 crossover and other gear and have it part-finance your new endeavor :)
Lastly: if I'm not incorrect the DSP filter topology applied with the M2's through the iTech's is with 36dB/octave slopes, which is what I use myself in my fully active setup. Moreover, some M2 users EQ them down low to extend to close to 20Hz, but that design natively is tuned to not much lower than 35Hz and so this is asking quite a lot more of its 2216Nd woofer. By high-passing them instead at, say, 80Hz or more you're doing the exact opposite by adding substantial headroom and making for a cleaner, more effortless reproduction. Win-win.