Impedence change with cables and sub

I took a look at the literature and the manual for the M51. It is not made clear whether or not the RCA and XLR outputs are driven from separate output stages (i.e., if they are separately buffered). If they are not (i.e., if the RCA output connector for each channel is driven from the same circuit point that drives one of the two signals that are provided to the XLR output connector for that channel), then connecting the sub to an RCA output would probably have adverse sonic effects on what is heard from the main speakers.

Putting it another way, it is not clear if the design is intended to support using both outputs simultaneously. Hopefully NAD can answer that question.

And in any event connecting just one channel to the sub will not be sonically optimal. The two channels should first be summed together, to produce a mono signal. A lot of people do that successfully with a y-adapter, but depending on the design of the specific component that might have some significant downsides and risks. Using a line-level mixer instead would be a better approach, IMO. See this thread for a discussion of that issue. But even the mixer approach may have undesirable effects on the main signal path if the RCA and XLR outputs are not separately buffered, because it would result in differences in loading and noise pickup between the two signals in the balanced signal pair of each channel.

Regards,
-- Al

I would recommend you stay with connecting the sub via the high level outputs from your power amp. In that case you will not have any impedance loading issues, and you also don't need to worry about the power amplifier gain vs the gain in the sub that may become an issue if you use low level pre-amp outputs. The sub should also sum both channels for you to get the mono signal Al is refering to.

Right now I have the hi level connected and it seems to work pretty well
So I will stay with that and avoid any issues. Thanks for the help!