trying to add a subwoofer to a Mac system from before subs were a thing….

Ok, I have a vintage system. All McIntosh.

If you want specs on these pieces, please check out the awesome Mcintosh Compendium here: http://www.mcintoshcompendium.com

Pre-amp: C-30

Amplifiers: MC7300 (two of them) 

main Speakers: McIntosh XR-7

Subwoofer: McIntosh HT-12 passive 4 ohm sub and without built-in crossovers. 

My goal: add a sub in a system without an LF sub output. 

Attempts so far: Using the "speaker B" output on the pre-amp, I've done the following:

1, I took the 2nd MC7300 and parallel mono'd it, sending 600wpc to the sub using home-built "Y" cables. The entire un-modified signal, full frequency was amplified. Having a single 12" woofer meant there was a lil gain in lower end volume, but not much bump. 

2, I created a low-pass filter coil. This has removed highs, high-mid and most of the mids. Now when I switch from "speakers A" (the mains) to "speakers B" (the sub), the mains have soooo much more volume than the sub. The lows are so quiet.

When the mains are on, I can switch on/off the sub and frankly, hear zero difference. Maybe my coil is too big. I can try removing some but the more I remove, the more low-mid to mid I'm revealing and my goal is to feel the bass, not just hear more. 

An idea floated to me was a MiniDSP between the pre-amp and the amp. Maybe this way only the low frequencies I want will be amplified. However, that seems like a complex solution and might stretch my audio intellect on the set up. Example;The word "latency" was tossed around. Had to look it up.

What would you do?