XLR vs RCA

A lot of times this has to do with the components being used. Like if a fully balanced device offers RCA inputs or outputs, and those are generated from the balanced signal being internally converted to single ended. Just to allow for more compatibility. That option often won't sound as good as sticking with the native balanced format. It is possible to handle that conversion well enough to be pretty transparent but a lot of designs cut corners here and it doesn't sound the same.

Not sure if this applies to your equipment or not. It could also just be that you prefer the Mogami cables to your other brand, even if they aren't as expensive.

XLR & RCA are simply connectors.   The important question is what are they connecting to?    Is it a truly balanced circuit - which uses XLR?  Or is it a single-ended circuit - which uses RCA?     

Are both components balanced?  Or is just one balanced? Or are neither balanced?  Or is the XLR connector on the chassis a fake balanced circuit.  The manufacturer simply daisy-chained the XLR connector to the RCA internally in the component. This is for convenience and to look cool    Homework is needed to determine what kind of circuitry your gear has.  It should be in the specs.

More info here.

From everything I have been able to find out, both the Linn and PS Audio a fully balanced.  But either way it sounds fantastic.

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.

What RCA interconnects were you using?