Rel R-505 work with a Pass Labs X350.5

Some powered subwoofers require an audio signal ground reference and their makers may suggest that a black speaker terminal is where this voltage reference might be found. On complementary designs, which would be typical of all power amps produced by Pass Laborato- riesTM, this is an incorrect and unsafe assumption. On all Pass Labo- My present amp is a Mcintosh 402 Magnepan 3.7
You should contact Pass directly on this.
Thanks I did had to get more power because it seems like Magepan is no longer going to make biamp speakers any more.
As indicated in the manual, from which you were quoting, the negative speaker terminal should absolutely not be used as a ground point for the sub, because the amp has balanced outputs and therefore a signal is present on that terminal. A chassis screw can't be used either, even though the REL manual might suggest that, because in this particular amplifier chassis is isolated from circuit ground. What Pass will probably suggest is a means of rigging up a connection to either the ground sleeve of an rca connector, or to pin 1 of an xlr connector.

-- Al
Thanks for taking the time to answer my post happy holidays
Contact Sumiko. They have a special cable you can order that has an RCA termination instead of the black speaker wire that uses an unused RCA plug on your preamp for ground (or power amp if you're going balanced). I believe it's $200 if it's worth it to you.

Also, you may want to contact the member Zikhmark, who sells an aftermarket REL Neutrik cable here. Perhaps he can make an RCA ground version for you. It would probably be cheaper as well as an upgrade to the custom speakon from Sumiko/REL!
It would seem that any magnepan speaker with a fuse and a seperate hipass / lopass crossover WILL be biamp able. Even if you must DIY.
My panels, the 1.6 has a lo pass for the bass and hi pass for mid/tweet. So, even if their were only a single set of terminals, It's still likely that anyone with a little DIY in 'em would be able to make the necessary changes.

The 'simple' test for common ground amp would be to turn off and unplug. Let it sit for an hour, just for safety to discharge any of the big caps. Check speaker ground to ground with an ohm meter. Should be zero from both speaker grounds.

I'd be careful with ANY amp these days.