So I switched from XLR cables...

I'd like to introduce a new thought in addition to almarg's comments..  Even though capbeaver's XLR and RCA cables were made by same manufacture, there could be a significant difference between them.  Are the XLR/RCA cables the exact same model?  Or different models.  Maybe the XLR is silver/silver-plated wire and the RCA is copper?  Are the XLR silver-plated?  Maybe the RCA plugs are gold-plated (which would be a much warmer sound than the silver plated).  Also, many cable brands use a basic Neutrik XLR plug.  This can have a slightly dry/harsh sound because they use hollow/thin conductors.  The RCA plug could have a solid center-pin and conductor element, which could dampen the electrical resonance that shows up in the low mass Neutrik XLR.

From manual:

Balanced XLR Input Jacks
In most systems balanced XLR connections will give you the best sound. If your stereo preamplifier has balanced XLR output jacks, we recommend that you connect them to the A 21+ XLR inputs. Refer to the Balanced and Unbalanced Lines in the Technically Speaking section for additional information about why we recommend using balanced lines.

Note: Using balanced XLR input connectors results in a 6 dB higher volume level compared with using the RCA input jacks. This is a noticeable increase.

Unbalanced RCA Input Jacks
Use these inputs if your preamplifier doesn’t have balanced XLR output connections or if you simply prefer to use unbalanced connections.

Balanced/Unbalanced Selector Switch
Place the switch in the position for the input type you will be using.

Note: The Balanced/Unbalanced switch is not an input selector. Its function is to optimize the signal to noise ratio for each type of input. You should not connect both the Balanced and Unbalanced jacks at the same time with the expectation of switching between two different devices such as a preamp and surround processor.

Balanced XLR Pin Configuration

The A 21+ XLR jacks conform to the industry standard of:
Pin 1: Ground Pin 2: Positive (+) Pin 3: Negative (–).

Great Captain, the Parasound conforms to the AES standard. Now how ’bout the Mac?

balanced signals require both halves of the signal propagation to be exact and then electromagnetically and in voltage fields..interact and reflect off/in one another in perfect symmetry of their perfect signals.

Nice theory.

Reality says it has it’s limits.

It gets niggling little bits of it wrong. It is not possible to make it perfect enough for the theory to emerge as a reality. *

Problem being that it’s distortion components are the parts that the human ear uses for fine levels of discernment...

....which is the whole high end audio pursuit itself.

Both balanced and single ended RCA.... both being imperfect...as well as the rest of the system and the humans being imperfect (different hearing, different mind, etc)..combined with how the human ego/limits function in the meat box we call a human...those things all combine into, or emerge as - arguments among the meat boxes.

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* The Teo audio liquid metal has the ability to deal with physical perturbations and inconsistencies in construction..whereas frozen lattices of solid wire, due to how signal propagates in it..cannot. In this.. the problem with wire in balanced design..is pretty well wholly eliminated with the Teo Audio liquid metal conductor pathways. This point is utterly unique in electrical conductor design.

Better designed balanced cables sound better when used with a truly balanced system, period. When I have the choice I always go for balanced. Otherwise, if you must use single ended (like with my Dennis Had Firebottle amp which has only single ended inputs) ICs they also sound fine of course, if you stay within the run length limits.