Thanks to all who posted. Here is an update. I spoke with E.J. at Wyred 4 Sound and he told me to use a multimeter to measure Ohms at the subs speaker level input.
1. Measure from Pos to Neg on eacdh channel
2. Measure from the left and right neg. to plate amp ground, which I was able to use the outer ring of an RCA input jack.
3. Measure from left to right neg speaker inout leads.
He said they should not be zero. Well he was right, the left and right negative posts measured 200 Ohms across each other and 100 Ohms to ground for each one. The hot leads measured OL to ground, and OL to each other.
That gave me the confidence to try the hi-level inputs from the amp to the sub..... well it worked fine. I had to lower the sub volume a bit from the line level I was using, but not a whole lot.
Now, to answer everyone as to why I wanted to hook it up this way. You see my passive preamp was splitting the signal to the sub and my amp, but the highs sounded soft and muted. I think the reason was because of the input impedance of the sub is much lower than the amp, and placing them in parallel would change the input imp. of my amp from 64KOhm to just a bit under whatever the Velodyne sub was. So I was using an input tube buffer to help keep the impedance up, but a tube circuit in the subs line seemed like a waste of time and money.
So now I'm comming off the amp and it sounds great. Thanks to all who posted.