Hi Anant,
I was a bit confused about the 521i myself, because after I wrote my last mail in a hurry, I realized that it was the 521 I heard and not the 521i. Then I needed to find out about the 521i and was forgetting (read lazy) to do that. Please do not get upset. I apologize for the delay in replying.
Are you saying 521i is significantly better than 5001
The Nad 521i CDP was the earlier model. I have not heard it. But from searches in the net, it seems a similar one to the 521 which I heard. In that case, yes, it got to be significantly better than the Marantz 5001.
are you saying my amp deserves even better than nad 521i?
Your amp deserves better than the 5001, no doubt. Actually, in addition I also would think it would not be a good match in terms of tonality. The HK6550 certainly has the potential to be paired with some quite good sources, no doubt. It definitely can take sources better than the 521, but looking at the age of the 6550, a CDP like 521 may be okay.
Any idea how to find about disk reading issues in an old NAD player, if buying remotely?
Answering this is not easy. If you have to buy an used CDP from out of town and therefore without inspection and with unknown history, you need a bit of luck for most CDPs, I would say it depends on the money involved, an amount that you are ready to take a chance with.
Again sorry for the late response and regards.