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

Actually you do want the mains playing bass. The easiest and best way to get really good bass is with a lot of drivers in a lot of different locations. Do a search for distributed bass array, DBA, or swarm subwoofer system. The two main speakers help contribute to that. What you really want is like this, five subs and two mains with great bass response. https://systems.audiogon.com/systems/8367 The last thing you would want to do is remove the bass from the mains and have just one sub try and do it all. Seriously. Hard to think of a worse idea. Leave your speakers full range, get a couple more subs. You will not believe how much better that will be.

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....

roxio, yes you understood me correctly. Looking at the Primaluna manuals somewhat poor back panel image I mistook that subwoofer output to be an RCA, my bad.

 elliottbnewcombjr is correct. The plus does have XLR and RCA Inputs that can be switched to route the signal through a high pass crossover. 
The manual doesn't mention that flexibility using the High Level Inputs.

Nor does it mention the use or result of a single high (speaker) level connection. I'm hesitant to suggest connecting one high (speaker) level cable from the integrated to any of the +/- subwoofers inputs since the manual shows only all four high level (speaker) cables connected.

Possibly someone can comment on using a second set of speaker cables coming from the integrated amplifiers speaker outputs to the Velodyne's High Level Inputs (page 17 Velodyne User's Manual)?

Unless the Integrated amplifier is having a noticeable undesirable affect driving your main speakers to there potential there may little or no advantage by using a high pass filter.
In my opinion the ability to use the Velodyne's on board Auto/Manual Equalization, Room Optimization, and Remote settings, would be far more desirable.

Consider the effort it would take to have a proper set of XLR or RCA outputs installed on the Integrated amplifier instead of a single high level output.
The Velodyne Plus possesses outstandingly useful abilities not found in most other subwoofer despite its design being being over a decade old. The Frequency Response and Parameters Screen (page 10,11 Velodyne User Interface Manual) is a huge advancement that should be made familiar to its user. 

High level sub inputs use a resistor to drop the voltage down to line level. Because of this the extra connections present no load and the amplifier can drive them just fine. If the OP would study my system, which is highly recommended, he would see there is even a picture showing this exact resistor modded inside my amp to turn it into a line out to drive my subs. 

Exactly what he wants to do. Only minus the unnecessary and counterproductive crossover stuff he's so determined to mess up with.

m-db and elliottbnewcombjr, thanks very much for pointing out the high pass output feature in the sub.

I think its possible to connect it up as follows:

(full range) Line level output from Dac => Rca inputs on subwoofer => High pass line level output => integrated amp => speakers.

I can use the volume control on the dac. Should be able to continue to use the EQ and room optimization in the Velodyne sub.

millercarbon, I will A/B the mains running full range v/s partial range in this new setup. In my existing setup where the Oppo does the bass management, setting the main speakers to "small" worked better.

I would just connect the subwoofer using its high-level speaker inputs connected to the integrated amps speaker terminals.  Adjust the crossover on the sub so it underpins the main speakers as well as adjust the sub volume control properly so as the volume on the integrated is adjusted, everything stays as one cohesive system.  Super easy and this is how I've been connecting subwoofers for decades.  Mains = full range.  Subwoofer = support/ supplement for the lowest registers below the mains.

I also run my speakers full range (down to 20hz) and reinforce the bottom end with an audio Kenesis swarm inspired subwoofer setup (two additional subs). The subs play from 16-80ish hz and integrate really well. If you can echolocate them, something is setup incorrectly (i.e., either volume or the sub's low pass filter cutoff).  

As you can see, there are lots of ways to play the game even with your current setup. Try them all and choose the winner. Do note that people tend to over cook their sub volume levels because they like to hear thump coming directly out of the unit. This ruins integration with the main speakers.

I've done both, with my Parasound HINT which has bass management system I can high pass mains and low pass the sub. Running the mains full range gives me the best full range sound with my sub. I like the bass from my mains so why would I cut them off?? You lose some dBs cutting the mains and a bit of detail from the lows as you don't want to hear anything from the sub but you do want what it adds to say the stand up bass.
I have the HINT low pass at 40Hz, volume at about 50% and phase at about 130 deg on the sub. I can put my ear about 12-18" from my sub and I don't hear anything, if I turn the sub off its the same but a bit less low end detail and volume.
My mains have 8" woofers so why would I want to reduce the amount of air I can move by only using the subs 12" driver?
I think running your mains full range is the best way to go.