Where is the weakest link?

"...Its a general question relating to cabling..."

I can't help you or your system if I don't know what it is. Generalizations usually don't work out...that might be a generalization but mine works.  

It depends. It is an unsolvable problem without knowing the components. I have addressed mine starting with digital interconnects, analog interconnects, speaker cables, dedicated line, power cords. But you have to have components of sufficient sensitivity, appropriately matched, before really knowing what to go after first.

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There is also the issue of how clean is your power and how much reactive stuff you have in your house.

Your system is a system. 

Heres system,

signal begins with a nucleus connected via usb carbon from audioquest to a chord tt2 dac, which connects via monster rca cable to a NAD M17 v2i processor, which connects to a mcintosh mc 255 via rca cable (sb balanced ?)..  

Am considering transparent super cables to connect to b&w 802 d3 mains and b&w htm2 d3 center.   

very appreciated, thanks

I have all Transparent cables and interconnects. They are excellent. Don’t know what you are using for speakers now... but probably a good choice.

You’ll probably notice the differences most in two channel since video, at least for me, is video is really distracting, so I don’t notice subtitle changes in home theater mode.

Also, I would consider upgrading the monster cable from DAC to Processor. This might result in a bigger difference than speaker... hard to be sure until you do it.

Once that is done, look at a power distributor / conditioner and power cords.

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Then look to installing a dedicated line.