At this level and cost you would have a NAC552 in place, Naim amps work best with a NAC.
I agree. The NAC552 is special.
NAC is by definition the “control” to the NAP.
Only one way to find out. Try it at a dealer
With and without NAC 552
For me the naim preamp is absolutely indispensable and part of the naim amplifier musicality
They are designed together ! The reason they - 552/500 are in 4 separate boxes is for power supply magnetic isolation.
Technically, this should be possible, as the ND555 has digital volume control for Apple certicification – if I understoond earlier debates correctly… And somebody in this parish connected his NDX2 to his 300DR… But he wasn’t convinced that it sounded any good. But beauty is in the eye of the beholder…
Seems risky to me, to depend on software only volume control, without a physical way to mute the output. If the software goes wrong…boom!
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This is thread referred above
Naim do not recommend using the ND555 directly into a power amp. The volume control is there purely to comply with Apple requirements, however it is not a substitute for a pre-amp in the amplification chain.
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My thoughts exactly!
In the (one would hope very unlikely event) of software VC failure defaulting to max volume, when you can leap to the rack to hit mute or grab the VC knob, you equally could leap to it and hit the power amp power switch - it is another physical way to mute the output in an emergency.
Edit: Not sure why this targeted HiFi Man
Totally agree … it could be quite a big boom !!
I had same discussion with a Nagra dealer…I told him that I have only one (!) source left and that a pre-amp could be skipped in my quotation. He replayed: NO way- he tried a 25k Nagra DAC w/ volume control directly to a Nagra power amp. That was not the same sound as the second best DAC feeding a pre-amp. His finding: buy the best pre amp even if you have only one source.
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I very much doubt it’s going to sound optimal and be totally safe.
For one thing the matching impedances are probably all over the place.
The input impedance of the NAP500 is 18k ohms
The output impedance of the ND555 I can’t find specified, however the input impedance of the NAC552 is 47k ohms whilst it’s output impedance matched for Naim NAPs is <50 ohms with a load impedance of 10k ohm minimum …
So to me I would expect the ND555 is optimised to drive a NAC load, not a NAP load… so it will work, but iit will most likely not be sounding any where near optimal. And if not optimal why bother with a 500 series product, or any Naim product for that matter.
I don’t quite understand why some forget the importance of impedance matching with preamps, which is a crucial activity for the amp to sound its best.
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The “mundane” volume control and input selection functions of a preamp are obvious. The signal’s characteristics are not. I think that the desire to “eliminate an expensive volume control and input selector” is what drives this way of thinking - ignorance.
Even Chord make preamps and they recommend them with their DAC,s like DAVE which have volume controls.
The standard reply would be, “Of course they want to sell us an expensive piece of (unneeded) equipment!” 
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Impedance/drive capability of course is indeed a crucial factor, and it may be that the ND555 isn’t suited to feeding a NAP’s 16/18k input impedance. However, as you indicated the information isn’t readily available, and I’m not sure that what information is available really supports concluding it is not.
In the absence of such indication from Naim I would certainly not expect the ND555 to be have the same capability as, say, Dave, with its very low output impedance and ability even to drive speakers direct let alone being quite happy and capable of driving virtually any preamp or power amp: However, the ND555 might yet be quite capable of adequately driving a NAP’s input, even if not intended that way.
Of course that is not to say that it wouldn’t sound better through a NAP as Naim intends it to be used, though I’m not sure that anyone has yet indicated an audible difference, let alone in what way.
Aside from that, with some streamers or stand alone DACs a different reason for not using directly into a NAP would be if they don’t have a low pass filter to remove high ultrasonics, which I understand the NAPs do themselves include, however IIRC the ND555 does do that.
I haven’t seen anything recommending that Dave be used with one of Chord’s preamps in preference to feeding a power amp direct, and certainly the manual for Dave makes no such preferential recommendation. Meanwhile on their website Chord says that Dave is “perfect with” followed by the names two power amps… Of course when anyone wants analogue input options as well as Dave, Chord would recommend using one of their own preamps.
Too late to edit, that should have read “the NACs do themselves include”.
From the preamp part of the website, they seem to be recommending their dacs, . Anyhow best to listen for your self. I tried to duck buying a 252/SC dr, with a Dave into 300 dr, sounded devoid of life, imo. But worth trying.
The same here, and Chord refers to Dave as a “digital preamp”…