Headphone demos from Audio Advisor--
Audeze LCD-X for $1,529:
Focal Spirit Pro for $307:
It is fair to say that they are in the same neighborhood, especially if you're getting a good seal with the 550. The NAD is a bit better, but it is very "iffy" to say that moving to it will be money well spent. Since you can't hear them for yourself I suggest that you check the reviews of both your can and the NAD at Inner Fidelity.
In the 1980's you could buy some excellent turntables in the general market. I still have a Technics direct drive turntable from that era. However without knowing the model number and condition of the JVC, there is little to go on. The turntable will the center of your kit. So it's very important that it provides a strong foundation for the rest of the chain.