The following item is compatible with the Denon DP-400: Audio Technica ATN3600L.

I have the Ortofon 2M Blue PnP MKII and am very satisfied. It's a lot better than the original pickup. And because it comes complete, it's very easy to fit

I have the Ortofon 2M RED and am very satisfied.

Certainly everything from the AT-VM95 series from audio-technica. This topic is discussed here, among others: https://www.analog-forum.de/wbboard.... In my case, a cartridge from the Ortofon 2M series has also delivered excellent sound experiences: Ortofon 2M BRONZE PREMOUNTED (cartridge)