I have the same amp as you (Marantz PM11s3) driving a pair of Raven Audio Celest' Towers (90 db 4 ohms). I'm in a 20X14 room with 7' foot ceilings and the amp has no trouble driving the Ravens. I like to let it rip once in a while and the Marantz has no trouble. As you stated the amp doubles down into 4 ohms and in less you are a total headbanger (which you are not) or in a very large room (which you are not in) in my opinion you should be fine.
As a side note the PM11s3 is a very underrated amp. I've had mine for over 6 years and for people that know me that is a world record. The PM11s3 was KI's final top of the line Reference Series AB amp, every thing after that was class D (which I've never heard, but heard very nice things about). The phono stage is outstanding. I use an Ortofon Quintet Black LOMC with excellent results. In my opinion you would have to spend a whole lot more money to get a better sounding SS integrated (and it may just be different, not necessarily better) especially one with as good phono stage. Your Pm11s3 is definitely a keeper.