The main questions to ask are ; are the RCA and XLR cables the same and what is the power amp that is connected?
I too have an ATC SCA2 amongst my preamps and I also have available two sets of long interconnects for connection to the speaker amps both Harmonic Technology Truthlink , one RCA/RCA single ended, the other XLR/XLR balanced. It is some years since I compared them but recall hearing no significant difference in sound quality so long as the volume control was used to equalise the output levels.
Perhaps I ought to go back and repeat the comparison but I am loath to undertake a re-installation. So, unless you advise me to the contrary, I assume that what you are hearing in part may be the difference between two different cable formulations.
The reason why RCA is louder than XLR is if the input sensitivity of the connected amplifier is higher for RCA than XLR or if the the XLR cable has a higher resistance than the one used for RCA ( or both). However If everything is working correctly the SCA has a higher output for balanced than single ended ( quoted by ATC as 15V balanced and 7.5V unbalanced both into 600 ohms).
Or are you in fact talking about the input connections? The RCA inputs on the SCA2 are more sensitive than the XLR ones so , yes, it will be louder.
NB: louder often seems to provide better sound even if it actually isn’t as there is a psychoacoustic effect at work. You need to use a meter to ensure that the comparative volumes are within <1dB of each other.