Where is the weakest link?

I think speaker cables can have the most influence on the sound of the system. Speakers are a very complex load with impedance that varies considerably over the frequency range. A quality speaker cable that is a good match with your system can make a significant impact on how it sounds. 

My sense is that interconnects need to be of quality but their role is greatly exaggerated.  Speaker cables are of more value.
The area between the router and the stereo preamp or the processor is a crazy mess of technological creativity; the good, the less than clear and insanely overpriced gear.  Connectivity issues are complex and frankly are a challenge to communicate and most seem to have a limited understanding.   Also the energy supply path between the wall outlet and the back panels of all the various components is also complex.

and we all attempt a best effort to address connecting the pathways from the wall and the router, and spend thousands to feel good about it.  Benefits at times are not really discernable. If i dont hear it maybe someone else will.  
I have seen so many pictures of systems and related equip lists.  So much variety.  So many brand names.  It can be fascinating as i begin to understand how all the pieces are fitting together.  I dont want to create an overly complex system.  So i try to be very prudent.  And have no idea when the buying will stop. It seems to go on and on and on. 
My not being married provides more freedom with my choices. Yeah. 

@jumia
“i have thick interconnects, some better than others, but not the cheap stuff, have spent alot on cables to speakers, including center.”
you’ve asked about priority in fixing the signal chain. 
Turns out those thick interconnects are monster cable. And your speaker cables based on a “spent a lot on cables to speakers” statement didn’t seem to be pointing to a weak link in comparison to the monster interconnects. I chimed in with my opinion based on details you have provided. You will connect a clean faucet to a sewer and drink from it directly or will you purify it first before drinking from that faucet? What did I miss?

@jumia - ALL cables are important, not just IC's and speaker cables.

But in order to hear the improvements I suggest tackling the cable issue  in the following order

1. Speaker cables
2. Interconnects
3. Power cables

That way you will be able to hear exactly what is changing

As for your statement

My sense is that interconnects need to be of quality but their role is greatly exaggerated.

ALL My cables provide the best ROI - better than any of my components ever provided - they are the most important part of my system, because they allow the components to work at their very best. 

As for which brands make a "real difference"

• try KLE Innovations cables - all of them (IC;s apeaker and power) are very good.
• If you want to step up a notch, try Nordost or Inakustik.

As for RCA (i.e. Single Ended) vs. XLR.

• some people prefer XLR
• others prefer RCA
• XLR has the advantage on long cables (over 30 ft) 
• otherwise well designed single ended will equal XLR performance
• I happen to use single ended

That's my "five thousand foot"  assessment - diving into the details of "what makes a good cable" gets complicated fast and most cable companies do not provide the details required to make a meaningful assessment of their products (for obvious reasons) so best rely on observations and recommendations from others.

Hope that helps - Steve