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

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.