Do Costly XLR Cables Make A Difference?

YES!!

I like to place my front end components on the side wall so they are not between the speakers and run long balanced cables to monoblocks which sit very close to the speakers. This allows me to use very short speaker cables, but means my Preamp to amp interconnects are 7M. My other interconnects are quite short (1M or less). I could not discern much difference between relatively inexpensive cables (e.g. Mogami) and more expensive cables for these short connections, but the differences were much more apparent with the 7M connections. 

I ended up settling on Iconoclast 4x4 UP-OCC interconnects. These are not outrageously expensive in longer lengths (now about $4k for 7M, was a bit less when I bought them) and provided a nice improvement in detail and a modest improvement in image solidity compared to other cables I tried. I didn't try any of the Uber-expensive cables though. 

And yes, my equipment (DIY) does adhere to the AES standard. 

@jaytor

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I could not discern much difference between relatively inexpensive cables (e.g. Mogami) and more expensive cables for these short connections, but the differences were much more apparent with the 7M connections.

I believe it. The most likely explanation is the interconnects you use between preamp and amp(s) are what influences the sound the most. I was in the same boat when I moved my front end components to the side wall and ran a 15ft Mogami XLRs to my amp for a month or so. It was not even close to what the AZ Absolute Copper sounded like. During that time I tried different interconnects and power cables on the source components and the Mogami were always the bottleneck in realizing the improvements brought by those changes.
I ended up moving everything back to be able to use my 1m AZs between the pre and amp again. Way more benefits using higher quality cables than removing a 5 shelf vertical rack from between the speakers. Now if the AZs weren’t so expensive for a 5m set it would’ve been a no brainer.

“Way more benefits using higher quality cables than removing a 5 shelf vertical rack from between the speakers.”
@audphile1

Exactly! I was using 25FT RCA cable between Pre and amp and always wondered about the weak link by settling on most affordable cable. I got around with this quandary by laying out my components horizontally between the speakers.

it will make a difference if the XLR cables connect 2 components that are both differentially balanced.