So there isn't any Bluetooth transmitter that could be added to the Marantz amp that would then send audio to wireless speakers and be better ie using the qualities of the amp, than just using wireless speakers and bypassing the amp?
You won't be using your amp, other than for switching sources, the wireless speaker will have their own amps. Also, if your only source is Spotify you'll be unnecessary routing everyone though the amp and then over Bluetooth which isn't ideal audio wise esp. if you can connect direct to the speakers via your Spotify app.
Obviously if you have other sources (turntable/cd etc.) plugged into the amp then it might be preferable, usability wise, to output everything to the speakers. In which case you could use a
Bluetooth transmitter plugged into the L&R pre-outs or optical out on the amp. But that's a) assuming your amp has such output connections and b) that whatever wireless speakers you buy support Bluetooth and more importantly that the L&R speakers can work together as a stereo Bluetooth pair (which isn't a given). It will also be, audio wise, a far from ideal setup
Also 'wireless speakers" are a bit of a misnomer. For a start, unless they are battery powered they will each need to be plugged into a mains socket. Secondly they tend to directly support protocols like DLNA/Spotify/AirPlay/BlueTooth rather than being designed to cut the cord (ie. speaker cables) between a traditional HiFi amp and speakers.
It's probably useful at this stage to clarify what sources you already have and what you actually need/want to play back to the wireless speakers. Also, what budget / kind of wireless speaker are you looking at?
Is there a reason you wanted wireless speakers rather than wired ones? If it's because you don't want to run speaker cables between your amp and speakers it's probably the wrong reason for getting them (as odd as that might first seem given the name). Most people use wireless speakers because they want to cut down on boxes and have a minimal streaming based setup, not because they don't want to run speaker cables.