[schugh]

Hi, I've just placed an order for a pair of REL S/510 subwoofers.
I will get them in a couple of weeks.

It appears there are different ways to connect them depending on the amplifier type.
I have the 7B-SST3. But I don't know if they are Balanced Differential Mono Blocks or regular(?)
type A/B mono blocks?

For the latter you can connect the negative of the high level cable from the REL to the negative speaker terminal on the amp.
But if it's Balanced Differential then you are supposed to ground it by maybe wiring it around a chassis screw.

[Frobozky]

I have the 7b sst2's. They are balanced amps. I connect them to Rel Tx9's. I twist the yellow and red ends from the Rels together and attach them to the red output on the amp. I leave the black wire from the Rel unattached. This works fine. Rel suggest if there is a hum to attach the black to the ground on the pre-amp or to connect to the chassis of the amp.