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
Al,
"So are you saying, then, that the only difference between what worked and what previously didn’t work was twisting the red and yellow wires together, while having exactly the same connections in both cases?" YES

"Or are you saying that the red and yellow wires are now connected to the same positive output terminal of the amp, while you wait for the second sub to arrive which will be connected to the positive output terminal of the other channel?" YES, per my conversation with Justin that's the way I have to connect dual REL subs anyway; therefore there wasn't any risk in connecting the single sub this way. He assured me nothing bad would happen, so I tried it. 
chazzzy007,
Thanks.

"Ricred what sub are you upgrading from and what improvements are you hearing with the Carbon limited? I have an S5 SHO." I had a JL Audio F112 v2. The REL Carbon Limited totally disappears compared to the JL Audio F112 v2. I'll make more comments in a couple of days.

@ ricred1 that’s why I asked if your amp was class d. I knew that had been an issue.  As you know I’m a huge fan of rel subs. I think they are the best. I hope you can get the connection you need. 
@ricred1   By summing the red and yellow wires together and using the black wire to negative you are only getting the signal from one channel of your amp. This is the way that I connect my dual subs...one per channel! But if you connect just one sub in this manner, you are only getting the information from the channel that you have connected to. IOW, a mono signal to the sub in question. This doesn't actually solve your problem if you want to use just one sub and have it run off both sides (l,r) of the stereo amp. ( or left/right mono blocks).