Impedence change with cables and sub

I have a NAD M51 DAC that is also my preamp. I use the balanced outputs to go straight to my amp and not the typical RCA's which it has as an option to output from as well. I just purchased a REL subwoofer and will probably start with the hi level inputs to the sub from my amp. However, should I not like that I would like to go with the low level RCA inputs from the preamp that arnt being currently used. However the REL only has 1 low level input jack, so if I use just one RCA from say the right channel on the preamp and dont use the left channel to supply the sub and I use the balanced cables to the amp as well, will the electrical impedence or any other elecrical signal differ from right to left since one channel (the right in this case) is loaded up? Not sure Im explaining this right.
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.

-- 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!