You are not alone. We have similar setup, but my preamp is the older version C2500. I got Bartok (upgrade from Chord Qutest DAC) in March this year. Similar to your expectation, I was excited that I finally have a DAC that has balanced output so that I can finally use the balanced input on my C2500. I initially borrowed a pair of XLR cables from the dealer while I was waiting for a new pair that I ordered somewhere else. However, the XLR connection disappointed me immediately, when compared to RAC connection (similar level of the cable). I would not describe it as thin or narrow, as the sound from balanced connection is still warm and comfortable, but it is about 10% lower in volume (I do not know how to translate McIntosh’s 10% volume to dB, but you know what I mean) and less dynamic/impactful (after I adjusted it to similar volume level). Initially i thought it could be the problem of the balanced cables that dealer lent me, but later when I received my brand new balanced cable, i heard the same, so I was able to rule out any potential cable issues. I then went through the exact same path as you did to find answer, and learned from McIntosh that C2500 is not fully balanced, and therefore I drew similar conclusion as you did. It was hard for me to digest why balanced connection sound worse than RCA. I struggled for a while and ended up letting it go. Since balanced connection did not sound bad, and its less impactful sound signature is actually good for longer listening, plus my belief that balanced connection should have better signal to noise ratio, I settled with balanced connection. Not ideal and not perfect, but I could not figure out why or any solutions.
My previous post regarding the issue, if you are interested to read: