Don't think my preamp likes feeding signal to two seperate places.

OTOH, I'll take that LS17 off your hands for no charge...

Almarg and atmosphere, you have it correct.  I believe the paradigm crossover unit is 20k, SE, as per my referenced thread. (That user had the exact same crossover, but with a ref 3. It seems I would need to have a balanced crossover, as the ARC doesn't like to have both the SE and the balanced outputs running together. Imho, equipment costing this much should be built with more robust or better implemented output stages. Many of us will utilize two seperate amplifiers or in my case an amplifier and a sub crossover. We shouldn't have to worry about the output stage "choking". The difference in sound quality once the crossover unit is disconnected is amazing.  I had no idea how much I had been missing before.  I will not connect the sub until I can figure out what I will do to remedy this shortcoming, because the bloom and air is too good to give up just to extend the bass. From Preamp to crossover, I'm using a set of 1 meter silver interconnects that were made by a gentleman for me that I no longer have contact with. I do not know it's capacitance. The signal from the crossover is fed mono to the 555se with a  no frills radio shack gold single cable which is two meters.  I would assume the capacitance of the radio shack cable, and the he input impedance of the 555se doesn't matter because the paradigm crossover is acting as a buffer itself, and that the issue is from the paradigm to ARC relationship.  

Many subs can operate off of the output of the amplifier terminals. You might contact the manufacturer and see if this is the case for your model.

The paradigm unit is a stand alone crossover, not a powered sub.  The paradigm is fed from via rca from the preamp, which sends a line signal to the 555se, which is used as the amplifier for my passive sub. I do not have speaker input provisions, unfortunately. 

The xover as you say is 20kohm input, this combined with the se input 150kohm of the DS450 is a total loading of 17.6kohm.
The owners manual states of the LS17 regarding the minimum load for it.
"20K ohms minimum load and 2000pF maximum capacitance."

It would be nice if that LS17 was around 100ohms output impedance or less. The next best is to get the input impedance of the xover changed from 20k to say 100k, it’s just a small resistor and usually doable (1/2hr work) if the first active stage of the xover is fet input, if bi-polar there maybe offset problems.

The capacitance you can measure if you have a capacitance meter, but I dare say your interconnects will be under the 2000pf max, unless your running cheap high capacitance very long lengths.

Cheers George