well headphone speaking I have listened to a stock 521,541,modded 541, and I have the 542. also heard the musichall cd25, and the jolida mentioned above.
to me what stands out with the NAD is there way of bass, they do bass like no other to me. I am not saying the boom boom but just the lows in general are amazing. the highs are there and so are the mids but not as rounded as the cd25's are. the nad's are very forward with there sound, while the jolidas seem to be more laid back.
I am having a electrical chasis thing with my nad now, I guess all of them have it..not a probelm on the sound or the quality/safety.
over all A great player for the price, and yes you can have them modded