REL B3 connecting to Linn Klimax Solo help...

Not sure what you mean by a ground plug. But in any event, proceed as I described in my initial response:

If there is a binding post or other terminal on the rear marked "earth ground," or similar, use a multimeter as I described to determine if that is connected directly to circuit ground. If it is, connect to that binding post or terminal.

If there is no such binding post or terminal, use the multimeter to determine if a chassis screw is connected directly to circuit ground. If it is, loosen that screw, wrap the ground wire from the sub around it, and tighten down.

If neither of the above applies, solder the ground wire from the sub to the ground sleeve of an RCA plug, as I described, with nothing connected to the center pin of the RCA plug, and insert that RCA plug into any RCA jack on the preamp.

Regards,
-- Al

Thanks Al, I understand now

Oh, how can I determine using a multi if its connected to circuit ground?? Ive never used one before.

The manual for the multimeter should explain how to set it to measure resistance. If it is the kind of meter that requires a particular resistance range or scale to be selected manually, set it to R x 1, so that the reading corresponds to ohms (as opposed to tens or thousands of ohms). There may be a knob on the meter called "ohms adjust" or similar. If there is, touch the two leads of the meter together and adjust that knob until the meter reads zero ohms.

Disconnect all interconnects and the power cord from the preamp, as I had indicated. Then measure the resistance between the ground sleeve (the outer part) of any RCA jack on the preamp and the "earth ground" binding post or terminal, if there is one. If there isn't one, measure the resistance between the ground sleeve of any RCA jack and a screw or any other exposed metal on the chassis.

To measure the resistance between two points you touch one of the meter's leads to one point while touching the other lead to the other point. If the measurement is zero ohms, or a very small fraction of 1 ohm, it means those points are connected directly together, and you can use the binding post or chassis screw to connect the sub's ground wire. If the measurement is not zero or a small fraction or an ohm, use the RCA plug approach I described.

Regards,
-- Al

Thanks Al, ill see how I go ;)