Krell FPB cx amp with Rel Studio III sub base sys

I need the correct wiring info for a Rel Studio III wired up to a Krell 700cx with a Synergistic Research phase one X2 Rel Spec cable which has the Neutrik Speakon plug and on the amp side they use two wires marked red and one black and on the stock cable they use red/black/yellow.I know I should connect the two red wires to the + speaker binding posts however can I connect the black ground wire to the - speaker binding post or should I try and find a connection on the chassis?
I just added a REL Stadium III to my Krell FPB-600C with a SR Rel Spec II hi-level cable. I contacted Krell about the wiring. They said to run the (+) wires to the left and right positive speaker terminals and the white (-) to one of the chassis screws on the amp.

This worked well until I ran an additional XLR cable to the REL from the LFE output of my processor for HT. I then got a pretty bad 60hz hum. Sumiko recommended disconnecting the ground wire entirely (tape it off with electrical tape). This eliminated the hum entirely and it works great for 2-channel and for HT. Only do this if you use the XLR LFE input on the REL in addition to the hi-level cable.

When setting the controls, remember to lean toward setting the crossover low (mine is set at the lowest setting on both dials (22 hz)) and turn the gain up vs setting the crossover higher and the gain lower. It will integrate much better this way.

I would recommend that you send your Rel Spec I X2 to SR for quantum tunneling and upgrade of the shielding to the new Tesla configuration. It makes a big difference.
I'm not going to be using it for HT right now so would be using your first wiring layout. So where on the back of the 600c did you choose to grab a chassis ground screw without scaring up the amp?
There is not really a great place as you have discovered. I would try it just taped (with electrical tape) to one of the rear handles first to make sure that hooking up the ground is the way to go. If you can live with that aesthetically, then just leave it that way.

If not, and assuming your Rel Spec cable came with spades, try one of the "star" chassis screws at the rear corners underneath the amp. They are short and beveled, so you may need to replace the stock screw with a slightly longer one with a button head at the chosen location to provide good contact without scarring your amp's finish.