I purchased a used Nap 300 back in the summer and found it to be a rather disappointing experience. It just didn't have the pep of even my modest SU. Wallowing bass and rolled off highs compared to the SU. I wondered why I had bought the thing! (purchased un-seen)
So I made enquires with Naim Technical and they (Phil) explained the DR upgrade and service process for the 300 and I decided that I would never be satisfied until I had given it a go.
So I booked it in through my local dealer and just two weeks after it went in it was back with me running my system again. I am very glad that I did because the amp now sounds just as I had originally hoped.
My experience of Naim service has been amazing.
Not had much chance to listen to it fully, but from first impressions it sounds very good.
Just thought i would share my experience, if your old amp has lost "a bit of spring from it's bungee" a service or an up grade may be what it needs.