None of the approaches that have been mentioned in the thread will harm your equipment in any way, UNLESS:
1) You connect at speaker level and the class D amplifier is one of a few older designs which operate both the + and - speaker outputs with a large DC offset from ground (something like 24 volts if I recall correctly), as opposed to the much more usual situation of the - output terminal being at circuit ground. I suspect that concern is not applicable to your amp, but it would probably be a good idea to confirm that with Nord Acoustics (especially if your amp was produced more than several years ago).
2) You connect at speaker level and the class D amplifier has outputs which are balanced or bridged. Again, it would probably be a good idea to confirm with Nord Acoustics whether or not that is not the case. If it is the case, and you connect the subs at speaker level (rather than to the preamp’s outputs), the ground (negative) wire from the sub should be connected to either a circuit ground point on the amp or to a chassis screw on the amp, rather than to the negative output terminal of the amp (or the negative input terminal of the speaker).
Regarding the question about whether to connect to the speaker terminals or the amp terminals, chances are it won’t make any difference sonically, so I would go with the speaker terminal approach that is more practical for you. And if it does make a difference, it’s even possible that the speaker terminal approach might be sonically preferable, as it might result in a slight reduction in the amount of RFI (radio frequency interference) that may be picked up by the cable and in turn injected into the feedback loop of the amplifier (if the amp has a feedback loop).
Regarding how to connect to subs having RCA inputs for two channels, given that your preamp appears to have a very low output impedance connecting to just one input, or using a y-adapter to connect to both inputs, or connecting to just one input and putting an RCA shorting plug on the other input would most likely all work ok from a sonic standpoint (and none of those approaches would create a risk of damage). Personally, I would go with the approach of putting a shorting plug on the unused input. You can find inexpensive RCA shorting plugs from various sellers at eBay, some of whom should be able to ship to your country inexpensively.
Regards,
-- Al
