REL subs with Rowland Amplifiers


I'm having issues connecting my REL Carbon limited subwoofer to my Jeff Rowland 625 S2 amplifier via the high-level input. I connected my sub according to REL's instructions...yellow lead to positive on one channel, red lead to positive on the other channel, and floating the black wire(ground). I get minimum output when connected according to the instructions. When I use the low level input, the output is sufficient and the sub sounds great. Per my conversation with Jeff Rowland I need to ground the black wire by loosening a screw on the amplifier and connecting the black wire, but if it isn't properly grounded I may damage my amplifier. Is there anyone that owns a REL with Jeff Rowland 625 S2 amplifier or other balanced differential amplifier? If so, how do you connect your REL via the high-level input. Is there an easier way to ground the wire than unscrewing the screw on my amplifier. I just don't want to unscrew the screw and prefer another method of grounding the sub.   
ricred1
I own a JR model 8T. I had the same type of issues as you. Having contacted REL, I was told to hook up the system essentially the same way, yellow on the positive left, red on the positive right and the black to the negative ( neutral) of the red channel. This seemed to be ok. However, after I had my amp modded by Jeff, I asked him about this and he suggested that we add a ground connection at the chassis of the amp...which he did. I now utilize the ground connection for black, and the red to right, yellow to left. You might want to send your amp back to Jeff for him to do the same thing. Not sure I would want to loosen a screw of the chassis as Jeff suggests. Nonetheless, I think Almarg's advice is also correct.
I'm just so surprised that the compatibility problem between REL and Jeff Rowland amplifiers and other balanced differential amplifiers isn't well documented.

lalitk,
Thanks so much for your assistance.

daveyf,
" Not sure I would want to loosen a screw of the chassis as Jeff suggests." I know it sounds trivial to some, but I couldn't agree more and I'm not going to loosen any screw. I may contact Jeff Rowland and ask about adding a ground connection on the chassis. 
 @ricred1  I’m not so sure that the compatibility issue with REL subs and balanced differential amps isn’t well documented. There are quite a few posts on this forum and others about the issue. REL themselves now has a helpful video on their web site about hook up protocol. 
I run dual REL subs with my Jadis mono blocks, there were also some issues to hooking up those as well. I ended up having two connector cables custom made by Signal cable. Took me months to get the blend correct, and last night I noticed that my left subwoofer seems to put out a lot less output than my right...not sure why this is the case, but more experimenting to find out what’s going on. Sometimes these subs can be a royal PITA.