What should I upgrade next?

What do you think your weakest component is? If not that, then what do you want to improve? Personally, I don't like your speakers. It doesn't really matter what I think, though. You're the one who has to listen to them.

I will say that your speakers and electronics are pretty well matched. It would be a good idea to focus on specifically what you want to improve. If you upgrade something without being sure, you can easily make a mistake.

Replace the Speakers, go for something bearable, Verity Audio for example.

Should your room be tweaked to your satisfaction than the speakers would be my next concern. When you begin to hear what your don't hear so to speak, it may be time to upgrade your speakers. Beyond that it may be that expanding your system into a computer based or media server component will bring more joy and more use of a already fine system.

At the time B&W wanted to shift (rise) the price of the big 800 series and they managed to justify by building a new tweeter. (this gave a +50% price rise).

I would keep the speakers, the diamond tweeter does not worth the difference.

on the other hand i would do something with the sources. I am a TT fan but since you prefer digital i would buy a DAC and rip my cd's in flac format. try downloading a couple of hi res tracks and you will hear the difference.

make that a DSD capable dac, plenty starting at relatively low prices