Coming from a NXS2 setup I thought it may help others to know that recording via the USB on the top of the X1800 is possible with Rekordbox. You have to download the Denon DJ X1800 Prime Asio (from Denon DJ) and select it in your Rekordbox Preferences / Audio / Driver
Set the input record to USB 9/10 and then either use the Rekordbox Record Level control (next to the record button!) or use the Utility screen on the X1800 to increase the output level to the desired setting.
I’ve always found this the most convenient way to record my mixes on Pioneer gear and figured others may like to know how it’s done until Engine is updated with similar functionality (hopefully!) I don’t believe you need a performance licence either to use the Record facility in Rekordbox