XLR vs RCA

I find Mogami is a highly underrated cable. 

I should clarify...

Just an FYI, the PS Audio S300 amp is a "true" or "fully" balanced design.  I have also noted that when connected to a "fully" balanced preamp, it sounds better with XLR cables.

I'm referring to connecting to the S300 in particular.  Paul McGowan (owner of PS Audio) has always been a champion of using balanced cables, to the point that I can't remember anything they have made in a long time that didn't have XLR connections.  

I had a very similar experience as you about 8 years ago. I had a balanced system, but had it wired single ended. Just for fun one day I decided to wire it balanced with some 30 year old mic cables that were probably less than  $30.00 each when I bought them and they had all been dragged through mud and beer and everything in between. The results were amazing. I will never go single ended again.

I try to stick with balanced whenever it is an option. Some components only have RCA and that's fine, if I can though I'll go XLR. In my experience it is usually the better bet.

For decades the standard interconnect was RCA. Somewhere around twenty years ago balanced designs started becoming common and balanced interconnects became increasingly common. Now I would say the default for most companies are balanced... some offering newer methods. So, I switched to balanced about ten years ago. 

Folks invariably end up with a number of extra interconnects laying around. Which can be useful. So, it is very helpful you standardize on what is mainstream... since that is where most component companies are going to but their greatest effort. So, for now XLR is it.