I had 804D with Naim amplification and I agree it sounds a little harsh. I demoed a few amps. The best that went with the 804D was a Vitus RI100 (now called RI101) for a clean but smooth sound. One stage further were amps from Luxman, whose sound is very warm, almost tube like. The Luxman lost some of the detail that the Vitus brought however.
Both were far better than the Naim 552 pre with a 250 power amp.
It is interesting that the Luxman C-700u / M-700u and Vitus RI-100 sound better than Naim NAC 552 / NAP 250. I wonder how the Luxman integrateds say 590AXII or 509X would compare to the C-700U / M-700u separates. Since you mentioned the Luxman separates did not sound as detailed as the Vitus RI-100, I presume the Luxman integrateds will sound even less detailed than the Vitus.
I have not listened to any Vitus amp before but the Luxman L-590AXII is very impressive, sounding astonishingly refined and detailed although it’s just an integrated amp. The L-590AXII actually sounded more refined than the Naim NAC282 and NAP 250DR separates which I also own when matched to some Marten speakers.