OP
your original post is to short on information. What is your current source for your digital files? Does your Marantz amp come with a streaming module? Does it have an included DAC?
If the answers to the two questions above are yes, then does the streamer-DAC accept Qobuz directly? Or are you using a PC that is attached to the DAC by USB? Or sending Qobuz from a portable device to the amp by Bluetooth or AirPlay?
Regarding your iTunes files, presumably everything in there is available in Qobuz.
If you ripped CDs or saved downloads into iTunes, and you want to keep a physical copy , just keep whatever device you are currently storing them in , and make sure that you have a backup. If you were to add Apple Music, then all your files will appear there.