How good can it get, really? - my stereo sounds amazing!

You have system hardware that probably couldn't be improved enough to make the extra costs worth it.  On the other hand, playing around with the listening area may not even have to cost much to bring about significant improvements.  Once you know how to control the room, you can then, perhaps, profitably experiment with some hardware changes (remembering that speaker changes will require further analysis of the room).  Cartridges are a good place to start--they provide a much quicker education on differences in sound than other components.  If you can't hear a difference between cartridges, you probably won't hear differences between other components.  Of course, cartridges interact with arms in many ways, so you may find yourself experimenting with arms, as well.  Maybe it's better just to sit back and enjoy the music. 

@davegardner OP
I feel the same.
In all, I am very pleased with my system, in my space.
My little pair of Maggies, very well integrated with a ADS sub and driven by a Cronus Mag II and meticulously set up, in a well treated room, sounds incredible. Whether standing, sitting or laying down, when listening late evenings, at moderate levels - the timber, placement and air seems just right and stays focused where it belongs - on a large stage behind and beyond the plain of the speakers 
Even though there are one or two things I would like to upgrade, I’m not sure that the upgrades would be worth the cost, or if it would even make an improvement ....Jim
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I have found my stereo sounds especially great after a bit of cannabis and a glass or two of wine. Even a modest system benefits from this tweak.

@hdbc3036  :-)

Have to admit - a cold brew or a couple of wines may have something to do with the enjoyment of my late evening audio sessions....Jim

I agree that you should just enjoy what you have. Having said that one thing that improved my enjoyment of my 805D3 speakers is one of the B&W subs. Namely the Db2Dwhich is stereo in one box. It includes built in frequency settings for their speakers.  The integration seems just about spot to me and no need for floor standing speakers. 
 
Cheers!