I currently feed my NAP 300DR directly by an RME ADI-2 DAC and it sounds very good. I have previously used a valve pre amp with no issues as well, although it would be good to check compatibility with valve pre amps because not all will work at their best feeding a power amp like Naim’s which has an 18k ohm input impedance.
Having said that 18k ohm isn’t especially low and unlikely to present a problem to most valve pre amps
I see no reason why that should not be fine - ideally output impedances should be many times less than the input impedance they are connecting to for small signal audio circuits - and that is what you have here… and also these are typically average impedances - they will typically vary across the audio frequency band - just like with loud speakers. This is quite different from transmission line impedances that should be matched along the line for a wide bandwidth.
I suggest you keep the controller volume low to start with - as you get used to how the levels match.
Who would have believed Denafrips would gel so well with Naim?! Hopefully, we’ll have another Naim believer within our ranks pretty soon In my opinion, music came to life after downgrading from 600W to just 80W.