The ls50s are great speakers that lots of people know about and are the type you can get lots more out of them with amp upgrades.
But if you are, don’t be sucked into this idea that hi res streaming is some kind of zenith of music quality that will transform a system. It doesn’t. In my view much of the added benefits of improving sound quality can be, on a pound for pound basis, achieved through amplification and speakers but in your case I wouldn’t advocate changing those kefs which are great. So up to a level, every pound you spent on a streamer would have less of an added benefit to overall sound quality than spending same cash on amplification.
Sources are sources which nowadays are all pretty good, and you could add something like a £300 quid sonos connect to your system, a £500 bluesound node 2, a £700 ish Cambridge Audio cxn or the £1k marantz you are looking at and in sound quality terms they will all be relatively similar. However if you could spend say £300 of the £1000 on a steamer and £700 on a power amp and use your intergrated as a pre and drive those speakers better (at the moment your amp is only outputting 70w at the kefs 8ohms whereas they have headroom to 100w), they will sound a lot better than any hi res format could achieve. Punchier bass, better slam and just more musically real. Greater power will give the kefs better dynamic headroom.
I think the issue you have is that marantz has a expensive power amp in the £2500 pm11s3, and if you wanted to stick with marantz and use your current amp as a pre. Then it depends if you want to stick with the intergrated or pre and power route. If the former, then I think you could use the money to change your intergrated to a better marantz intergrated. There are also great amps around £1500 which would be an improvement on your amp from likes of primare , Hegel, creek, cyrus, rega etc. Whatever sounds good to you with the kefs for your taste. If you really like the marantz sound then I think I’d upgrade to a better marantz amp, and bung on a relatively inexpensive streamer like a sonos connect. If you sold your amp for say £250 and put another £1250 towards an amp, you’d be going into much better amp territory.
another option is to buy an intergrated pre amp with a streamer built in that has better dacs than your marantz. The Yamaha pre amp streamer springs to mind, but I’m sure you could do better. Look at chord dacs? You’d just use the marantz as a power amp slave bypassing it’s dacs.
i still think though the best option is probably a new intergrated amplifier designed to work and perform better as one and inexpensive streamer added on, to get the best of both worlds. The idea of a new dac in a pre amp is a good one in theory but often in an intergrated it’s all about how the pre, power and dac stages all mesh together to get the best sound, which often work better sometimes as one, than separate intergrated and pre and power. Ie use dedicated pre and power or dedicated intergrated if you can. Also a lot to be said in same with similar make pre (with dac) and power amps. The marantz range doesn’t seem to be built around the same kind of pre and power ideal of cyrus or naim etc for upgrading boxes and building a system of pre and power amplifiers, if that’s what you might want to do in future, so it begs the question that if marantz don’t have a much better sound to your liking in a different better intergrated up the range on listening, someone else will.
Hope this helps.