First Foray into XLR

Why replug all cables in a system before auditioning new ones?  

To establish a baseline fresh connection. Unless a connection is gas tight, contaminants in the air can degrade the sound. 

Replugging should abrade these contaminants and restore fresh connections.

New connectors can have manufacturing 'goo' / lubricants, so it's always a good idea to polish well with a clean cloth before plugging into your gear.

DITTO fuse holders! and any other unsoldered internal connection e.g. crimped jumpers so popular today. Once the warranty is up, I solder them!

I always did and still do, only to a point, believe that the main reason to choose XLRs over RCAs is to reduce noise, when long cable runs are necessary.
Recently, while doing some A/B comparisons, with a friend, between a PrimaLuna, Dialogue, Premium HP, tubed power amp and a VAC, Phi-200, tubed power amp, I had a mind changing experience. We intensely listened and compared both amps through a pr. of Wilson Audio, Sophia-II speakers. We listened to the PrimaLuna, using the EL-34 tubes and the KT-88 and KT120 tubes. While the EL-34s sounded smooth and warm, the PrimaLuma needed the KT-88s or KT120s to come close to the overall, dynamics, power and presentation of the VAC. While the PrimaLuna, sounded very nice, indeed, it was not quite on par, in terms of dynamics, detail and overall scope of the stage, with the much more expensive VAC. Then we decided to run the VAC through it’s XLR, balanced circuit. I thought nothing could be better than what we had already heard, but WOW! Switching the VAC to XLR was mind blowing. We switched back and forth several times, ea. time concurring what we heard.
As mentioned by others - running XLRs, if your amp’s XLR posts simply tie into the RCA circuit and the amp doesn’t have a dedicated, fully balanced circuit - will net no gain. I’m now convinced, however, that with a very good amp, having a true, fully balanced circuit - I would choose the XLRs...IMO...Jim

From my observations many people mistake the 6db boost you get from xlr to be "better sound".
I know I did to start with...lol.
You would need to level match the rca and xlr inputs ( if possible) to draw a more realistic comparison.

Switching the VAC to XLR was mind blowing. We switched back and forth...

Unless the output signal is verified within 0.1db across the whole spectrum, back and forth switching is rather pointless.

If the cables are not identical, then cable deltas could swamp the electronic.

Different connectors of the same type with identical cables can be audible.

Uncontrolled variables have been skewing results since time immemorial.

@ieales We did not simply do A/B switching (I am not a fan of simple A/B switching). We extensively listened to both amps and to the VAC in both RCA and XLR inputs, at various volumes, and assorted genres of music, experimenting with two different, quality RCA ICs - a .5m pair of CablePlex Silvers and a 1m set of low capt. high res 99.9% copper. The result (even when compensating for volume) was consistently, a wider, deeper stage, with more air around individual vocals and instruments, when the Vac was running through it’s balanced circuit.
With your logic, nothing is, or sounds, any different than anything else - it’s just a matter of volume.
Even when adjusting for volume, we did not expect to hear an improvement on the VAC’s already incredible performance, but we did...Jim