Do upgraded LAN cables/purifiers/switch box make a difference. What is recommended, if so?

US $750 Hmmm....

Your components are very nice already. Here are two suggestions:

• Do you drink? If so, spend a chunk of that money on the best whisky, calvados, wine, or whatever you like. A few sips of really excellent stuff will improve the sound.
• Consider asking GIK or ASC or another vendor for an acoustic treatment plan. That usually involves paying a small amount (say, USD 100) that can be applied to buying treatments. Then, perhaps start small by getting what they suggest is most important. If you hear an improvement, keep going. If not, you’re not out very much. Usually, acoustic treatment will make improvements 1 to 2 orders of magnitude greater than cable changes, IME.

(For the non-scientists: 1 to 2 orders of magnitude = 10× to 100×)

@mike_in_nc Thank you. And yes I love my single malt Scottish Whisky (note without the e). Good idea to grow the collection! With a dram of the good stuff, everything sounds better - it's like a veil is lifted from my ears 

I also agree with your approach of slowly, slowly and 1 item at a time

I'll reach to GIK / ASC for some assistance. Hope they can work remotely - otherwise need to look for someone similar in Dubai - not holding my breath on that. I have used the Anthmen ARC to do initial room correction (without any acoustic treatment except for a nice rug, artwork, wall shelves for vinyl and of course furniture) The room size is awkward, with the length of one of the walls, being floor to ceiling full windows for a view of canals leading to the Arabian sea. I do close the roller blinds sort of halfway when I listen to music. 

I too would think upgrading the dac and streamer is money better spent. I have a LUMIN U2 mini and looked into the separate power supply. All reviews said there was significantly less noise as a result. So, make sure your future dac and streamer have isolated or separate power supplies perhaps. Seems like that’s where a lot of noise comes from. Good luck and very nice system. Very revealing so might benefit from improved power isolation. 

@devinplombier 

Your ability to contradict yourself in a short paragraph is prodiguous. So if bit perfect ethernet doesn‘t eliminate EMI/RFI how can a conversion to optical defeat it?
A little hint: because EMI/RFI travels along the bit perfect data on the ethernet and has no means of transport in optical DOH! Hence different cables suffer from different susceptibility for EMI/RFI incursion and filters on ethernet have a task to accomplish!

And btw: depending on the quality of converters chosen, optical conversion has its own problems…

Think in future about making apodictic statements that are patently false!

Noise and timing are the two main issues affecting streaming quality. Take care of your noise issues, lower noise floor means higher resolving/transparency. Take care of your timing issues, far closer to a continuous analog wave form, this results in more analog like sound. As for the equipment, cabling etc, everything matters, any weak link in chain affects the entire chain. 

Now as for where to start. So lets say you go for dac first, problem is if you have crap going in dac is not performing to highest potential. You can also start at the beginning of chain, so feeding dac with lowest noise floor, most precise timing, if dac weaker link not getting full potential of feed to it. 

Point is I've found every single link in chain important, just a matter of priorities. I'd suggest starting with dac and streamer as the most efficient way to achieve high  quality streams. Spend absolutely the most you can afford here, tweak away for everything else. As for tweaks, I've found purpose built audio components superior to the generic stuff. LPS for every single tweak that requires AC quality fiber conversion or fiber service great for noise reduction, don't cheap out on any of this. Finally timing, I've found master clocks are extremely critical for trying to achieve a continuous analog wave form such as found in analog souces. I personally will never purchase another dac without I2S input, or proof/evidence that dac master clock and power supply to that clock are of highest quality.