Ideally the black wire from the sub should be connected to a circuit ground point on the amp, but it appears that the Rowland 8 does not provide a binding post or other terminal corresponding to its circuit ground. (There are some balanced amps, such as some of the Pass models, which do provide such a terminal). Given that, connecting the black wire to a chassis screw on the amp will most likely work well, and at least some of the REL manuals suggest doing exactly that when connecting a sub to the outputs of a balanced (or bridged) amp.
Although it is unlikely, if doing that results in a hum or some other issue, and if your connections to the amp's inputs are balanced (so that its RCA input connectors are not in use), you could solder the black wire to the ground sleeve connection point of an RCA plug, leaving the center pin unconnected, and insert that into one of the RCA input jacks on the amp. That would establish a suitable circuit ground connection.
The issue that I indicated I was envisioning, given that on a balanced amp the negative as well as the positive output terminals have full amplitude signals on them, is that depending on the internal grounding configurations of both the sub and the amp connecting the black wire to the negative output terminal could result in a near-short being applied to that output. For example, if as is commonly done both components connect their circuit grounds to their chassis through a low resistance, by connecting the black wire to the negative output terminal a path would be established from the negative output terminal of the amp through that low resistance in the sub to the sub's chassis/AC safety ground, then through the AC power cords of both components and the external AC wiring to the chassis/AC safety ground of the amp, then through another low resistance to the amp's circuit ground. Voila, a near short will have been created between the amp's negative output terminal and the amp's circuit ground! And perhaps over time the resulting excessive current flowing through those resistances, and the resulting heat, have caused one or both resistors to become lower in value, or even to short out completely, increasing the load on the amp and causing it to heat up more than previously.
In any event, even if this is not what turns out to be causing the problem, it would be good practice (and perhaps sonically beneficial) to connect the ground wire as I've described.
One final point: You might have to readjust the level control on the sub a bit after changing the connection of the black wire as I've described, to keep the sub and main speakers in balance.
Good luck. Regards,
-- Al
