I feel the key point on this is if your not happy with the audio sounds of your av receiver, you have a few avenues to look down which is nice. Improving the source so you do not have to hear the processing of the av receiver, upgrade the av receiver or add an additional amp to enable you to bypass the av processing altogether
I don't think any of these are wrong and its up to you as to which one is the easiest for your to run down. If your av receiver has the ability to work with a dedicated stereo amp and your stereo amp can work as a power amp via a bypass option, then I feel you'd be a fool not to use that option. I on the other had have a half decent av receiver where I don't rely on its processing for stereo (it runs in direct stereo mode which switches the processing off) by improving the source. I've also been lucky enough to try this option plus its let me see how my dac and av receiver stack up against the Lyngdorf 3600 and I was super happy that the sound difference (with RP set to bypass) was almost identical
So adding a stereo amp with a bypass feature to work in conjunction with your av receiver (so long as it has the outputs) will be a great starting point, but its also relevant to ensure that the source is also as good. The reason behind this is it doesn't matter how good your amps are, if the information being sent to them is poor, then you will enjoy poor quality sounds until you sort it out. So its usually more than just upgrading one section in my opinion. Now the beauty of any system only needs to cost what you feel is a "good sound" as each and everyone of us has a benchmark we aspire to that end is all that's each one of us are striving to achieve
I've never heard the latest Nad av receivers, but historically they used to be very good. I've always preferred Arcam as a one stop av receiver as my ears enjoy the sound (Some people can find them to be a little slow, but I find that with my DAC and Media Server, plus the av receiver running in direct stereo is quick enough for my ears).
All, I will also add that if you're looking at cd playback, then see if you like the av receivers processing over the cd players. Try using both the analogue and digital outputs from the cd player. If you are using the analogue option from the cd player, remember to place the av receiver into its purest direct/pure mode which should switches many of those unwanted features off when listening to music
If you are streaming, ensure the information you are streaming is of a high quality, if not, then like I mentioned earlier, "you will enjoy poor quality sounds". So, make sure you find the right one for you. Try different options available to you as Airplay or Google own variation do sound different to many other gadgets like TV's and cheap connected devices (That's what many people have mentioned on these forums)
As you've mentioned an Arcam, I think you know my thoughts on that as I already own an old Arcam