@stereoisomer I think you articulated yourself well in this thread. The way I conceptualize engineering and the audio reproduction hobby is that we are often making compromises rather than across the board improvements--getting one benefit by giving up another. You articulate two options here. But, I see a third option given your gear.
Now I have introduced that subwoofer pathway into the signal path between preamp and amplifier? I'd love serious explanation on what that did, if anything, to the signal. My hope was that it was going to make the subwoofer and amplifier more in sync than if I'd run a separate connection from my second Preamp Out to the subwoofer. This way it's all in one signal path. So, is this the optimum way to set this up?
You consider either (1) running the sub and mains power amp out of separate preamp outs, vs (2) adding the sub to the chain so that it the mains power amp receives only the higher frequencies. As you and others have stated, both are viable options. The third possibility (3) is that you keep the sub in the signal path but defeat its high-pass filter.
Looking over your sub's manual, I see that you can defeat the high-pass filter by "by rotating the crossover frequency
control fully clockwise."
https://pdf.crutchfieldonline.com/ImageBank/v20150208141700/Manuals/877/877SDS12.PDF
You state that your goal was to "make the subwoofer and amplifier more in sync." Like you, I am also sensitive to the lag time of the bass frequencies. In my system, I use (1) because it's the only option with my sub, integrated, and room. It's better than no sub, but the sub frequencies arrive late. (One writer on the Darko forum states that latent subs are common, but certain brands make faster subs to remedy latency. See
https://darko.audio/2021/06/kih-89-mind-the-gap/.)
So, I think (2) may help you to achieve your goal better than (1) by placing the sub prior to the mains power amp in the signal path and getting those sub activating earlier vs the mains. However, I believe that (3) is also worth a try because, in theory, you may retain the sub's priority in the signal path, but you can have the added benefit of allowing the mains to produce sub frequencies too. That is, you have a multi-sub system of sorts with better timed bass. The downside to (3) may be that, with your specific sub, defeating the high-pass also requires your sub to play the entire range from 30-150hz. So, you might find that when overlapping frequencies with your mains, you may get too much sound from 60-150hz, say, and not enough from 30-60hz.
Tying back into the initial thought about engineering often being a game of compromises, I see the two prevailing sub setups as compromises. Connecting subs via speaker cable allows for better integration with the mains, and allows for a focus on sub latency. And connecting subs via RCA gives up that integration with the mains for smoother bass response in each seating position and across more
seating
positions, and can free up power on the mains amp.
