Seems tough to ensure only subwoofer is playing bass frequencies (& not the main speakers)

I would not mind using multiple subs of the same type for that purpose.

In this case, the passive woofers from the ported main speakers produce qualitatively different bass than the high powered sealed subwoofer.  I'd prefer to let 1 or more subs deal with bass... and let the mains deal with the rest.

Have you tried using an RCA cable between the integrated’s single subwoofer output to the Plus’ RCA (Right) LFE Input and using the Plus’ Line level control to adjust the subwoofers volume? Or to the RCA (Left) Input?

I use L/R XLR preamplifier outputs to a DD Plus which is connected to a second Plus using the RS 232 terminal.

The integrated’s subwoofer/mono output is actually a speaker level output. The RCA (right/left) LFE input on the subwoofer is a line level input. Don’t think its possible to hook them up (if I understand you correctly). My integrated does not have pre-outs. Using pre-outs wont help as the full range signal still goes to the main speakers.

that sub has ’output high pass’

https://www.velodyneacoustics.com/digital-drive-plus-series/

the sub grabs the bass, and crosses over at either 80hz or 100hz as you choose.

you send full range separate left and right signals to sub first, then it sends separate left and right signals without the low bass back to the preamp/integrated amp/tape loop, lots of options, then to your mains.

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Self-powered Subs: two advantages

1. amp does not need to amplify bass, so a lower power amp can be used, an important consideration regarding cost, weight, heat, and especially allows you to try less powerful tube amps.

2. mains do not try to produce bass that they can only do 'moderately', so you get the main’s mids and highs mixed with the sub’s bass capability.

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2 channel system, sub always on presumably, low bass always to the self powered sub, high pass to mains.

in addition to above, my old velodyne also allows speaker wires from the amp, thru it’s variable equalizer, then speaker wires without the low bass to the speakers. mains drivers do not have to try and do bass they cannot do well. no advantage to the amp, it still gets full signal from the preamp, amplify s the bass prior to sub.

I am in favor of a stereo pair of subs, forward firing, located next to the mains, producing primary bass notes and all the overtones of those notes, thus the bass is directional and improves imaging, i.e. where is the jazz bassist standing?

also, I am no fan of ports, passive radiators, .... again, to preserve bass imaging
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multi-channel/home theater, a single self powered sub is often enough, just to add the dinosaur stomps, black hawk down heliocopters ...

you want your full range signal going to the mains, then, in the av receiver, you tell it what size mains you have. multi-channel it equalizes what it sends hither and thither, however

often 2 channel, or direct sounds better than multi-channel, without the sub. In those cases you want your mains to receive all the bass, and do the best they can without the sub.

In this case, the passive woofers from the ported main speakers produce qualitatively different bass than the high powered sealed subwoofer. I'd prefer to let 1 or more subs deal with bass... and let the mains deal with the rest.

A tough sell. aka stubborn. They ALL produce qualitatively different bass. The exact same sub produces qualitatively different bass simply by being put in a different place in the room. Did you not look at my system? Three different types of subs.  

You can lead a horse to water....