To test I also used:
1. TR (L+R) to RCA - signal significantly improved
This gave much better signal but introduces digital noise. I assume because balanced to unbalanced
Do you hear this noise? Or are you just concerned that theoretically there may be more noise?
What do you mean by digital noise?
Balanced connections were developed primarily for pro-audio where you are running cords 50'-100' across stages and never know what sources of interference/noise may be present at different venues. Obviously any band going on tour would quickly get tired of having to chase down noise issues!
If I had audible interference at my desktop, I would want to at least make the effort to find/remove the source of the noise before deciding to work around it. If these cables are picking it up, it is probably a good bet thatit is effecting something else as well!
Are you running these cords adjacent to speaker or power cables (like zip-tied together)? That could also do it.
Inside of PC is notorious for noise, but the computer case should contain it.
Also, just to cover all bets, this isn't 60Hz hum is it?
But congrats on a nice system! I'm not sure you can specify it as a desktop system! It does a great job in that capacity, but I talked a friend into getting a system almost identical to your (different subwoofer) instead of a sound bar in her living room! She loves it! Just a FYI, she especially loved it when I explained to her that it is almost impossible to blow up these speakers! Her boyfriend and her like to throw parties where much beer is drunk and the volume knob often gets abused. One advantage of a mass produced monitor like this is the amp is designed/selected for the exact driver it will power which allows use of a limiter at the point where the speaker might be blown or where the load from the speaker would put the amp into clipping. It might not completely eliminate concern over damage from a "determined" idiot, but it is reasonably idiot-proof assuming "casual" idiots!
A wholly decent system by any measure; but for the price, this is makes for a killer system!