You’d use a Din cable from the 552PS, not the 552 itself. It would need to be suitably slugged with a 100R resistor.
I can’t se why you can’t connect to the back of the speakers just because it’s a 500, and have not heard of this potential issue before. If you’d prefer this connection, I’d get in touch with Naim to set your mind at rest.
You should be fine with a low level connection from the pre as long as you use a slugged cable (or a very short one) connected to the spare DIN output on the 552PS.
For a high level connection, which may well sound better, I believe Naim used to supply a special cable as the bridged design of the 500 cannot have a grounded sub connected to its ungrounded output. I’m not 100% sure how this cable is configured, Naim dealers should be able to provide one that is suitable.
I’ll probably play it safe and connect via the 552PS. Don’t suppose anyone knows whether the resistor should be at the preamp connection, or the n-sub connection, or no matter.