I wouldn't worry about a high pass filter with Dyn 40s (unless you listen at very high levels).
I will try this first.
...trying the easiest way first........always start with the easiest and most available/logical.
Thanks for the reminder.
Also, this can make sense....
One reason for a 2.1 system highpass, is to help relieve the main speaker from potentially struggling with too much low-end movement, and then the Sub take (cross) over.
But if the main speakers never get stressed by playing loud or hard.......then why add a filter if it's not needed ? .... just find filter heaven by adjusting the (adjustable) Sub .
It's a plan.
So for right now, PLC output #1 will go directly to the C100s amp to stereo speakers.....acting as if there is no Sub.
And #2 output goes to my Sub. And I try to adjust the Sub for best sound (crossover, placement, and volume). That may be enough.
Tests will tell.
......but, I'll eventually need to know what it sounds like filtered too, for comparison. (in comes a temporary passive filter)
I'll have to find that online filter calc...... and solder one or two up.