The N-Sub is an active sub, as are the vast majority of subs intended for home audio and AV use. If you can get it integrated properly into your room, which is not always so easy, it can be a great addition to any system.
Be prepared to take some time getting the setup right, otherwise it can ruin an otherwise great system. Keep an open mind about location. It’s a heavy beast but you’ll want to move it around the room to see where it works best.
Yes, you can use a high level connection from the speaker terminals. You just need some thin speaker cable, so it’s cheap.
Alternatively use a low level connection from the preamp taken from a spare socket on the Supercap would work.
If you are taking a low level pre-out signal from the PS then you need to make sure it’s either the specific Naim sub lead (was available in 5m or 10m lengths), which contains a slugging resistor, or one that is similarly slugged with a 100R resistor, otherwise it can result in instability from the pre-amp.