As a new SE84UFO2.1 owner I read with interest the advice in the manual about the floating ground and REL subwoofers, because I noticed a very loud hum from the sub on first hookup—even when the amp was off and the REL on. Both the amp and the sub worked fine independent of each other. I have rudimentary electrical knowledge but I can read—so I hit the Forums. I organised my cables; ran DC blockers to each item; checked all other components and tubes. It was definitely the hookup itself!

After a lot of fussing around this is what worked for me:

I ran a grounding wire from both negative speaker terminals and wound them together. Then I used an alligator clip to attach that joined wire to a lower screw on the transformer. That lower screw has some metal thread slightly protruding so it’s ideal.

However, ONLY the right hand side (looking towards the rear of the amp) transformer screw worked to eliminate hum. Obviously this is the ground for the amp. I discovered this by touching here and there with the connected alligator clip, looking for a point that would reduce or eliminate the hum.

I realise there are probably more elegant solutions, but I feel so happy about my success I am posting this information for other REL owners .

Sorry, the RHS lower transformer screw closest to the front of the amp. Most accessible and easiest to clip to.

Did I mention that I LOVE the musical balance and sound of this amplifier even while breaking in?

Here are some pics as requested (I figured out how).

The alligator clips at each negative speaker terminal are larger then the clip at the transformer screw. There's very little spare room at the back  so I needed to really make sure that nothing was touching where it shouldn't.

The arrangement is 'temporary' as there's no soldering nor taking off the bottom.

I did try running both negative terminals together but it didn't work for my system.

I forgot to mention that I have a power and voltage regulator and a dedicated line from my switchboard, as well as the 8 ohm- & 98dB- sensitive Tekton Design Perfect SET 15 speakers. The sub leads and cable are well-insulated. I'm never giving up the REL S/510, either.

After a couple of days like this there still is a '5% hum' remaining (according to my wife's acute hearing) which is inaudible to both of us more than a metre away from the subwoofer; it doesn't bother her, and it doesn't bother me (any more than 'tube rush', heavy metal music or occasional traffic noise does). And I am easily bothered by hum.

So far so good.