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

I am always amazed as well. Amazed quite honestly at how awful most of the rooms sound. Most of the rooms are poster children for the need for proper room acoustic treatment. I know that is harsh, but I always get a kick out of the "Best Sound" awards ... of which the number seems to grow every year. How many "best" can you have?  

Check out an audio show like Axpona or one of the other regional shows to see how other systems sound. I am always amazed when I go.

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