I asked REL directly about subwoofer connection to Naim amp. They do not recommend taking signal from speakers. Here is their answer:
Naim uses a star grounding system as every input is isolated. This can result in a mains hum showing itself.
Naim suggest coming of the speaker itself to work as a get around - which does work however it will be a mono signal and won’t have the individual “flavour” of the amp.
We always suggest coming off the amp whenever possible and no - it won’t have extra load on the amp - it doesn’t even know the sub is there.
We supply a hi-level cable in the box but we do a couple of other options that may be of interest:
The Baseline Blue - our upgrade hi-level cable. As it is fitted with spade connectors you would have to change the termination- probably best to fit a stackable banana plug.
Bassline Blue Naim cable - as above but also has a built in resistor to negate any hum issue. This comes fitted with stackable banana plugs ready for Naim already
often using the relevant wireless adapter for the sub cuts out any hum issue . For the new S products you would need the pending Airship product.
REL May say that to sell their sub, but it’s the Naim amp that could potentially be affected, so it’s far wiser to follow what Naim say. The comment about the mono signal is wrong - you have two of the three wires to one speaker (positive and negative) and the other positive to the other speaker, thereby getting a stereo signal.
Of course, when the preamp has both a pre out and a variable level sub out, you can take a low level connection anyway. It’s only the Unitis that are compromised like this but they are designed as one box systems without full flexibility.
In my experience, the Atom provides ample bass. I have mine running a pair of Gradient Evidence II speakers.
With that minimalist combo, I can make my rib cage vibrate playing ‘The Inexorable Advance of Mr Delaney’ by Max Richter. And that track comprises strings only!