Where next for my system?

[thumb5]

Don't rule out the digital room correction option too quickly!  I expect that would give you a much more significant improvement in sound quality than buying an external streamer, and for a fraction of the cost.  In fact, it's so (relatively) inexpensive that it's worth trying anyway.

I have found Focus Fidelity Filter Designer to be very easy to use, and it produces great results with only a little effort.  It works very smoothly with Roon.  (I don't have any affiliation with Focus Fidelity other than being a very happy customer.)  If you want to try this out you will also need a calibrated microphone such as the MiniDSP UMIK-1.