Many professional amplifiers come equipped with combo input jacks, either XLR or 1/4" TRS. The XLR and TRS inputs for each channel are wired in parallel.
My testing indicates that if you use the XLR connectors here, you frequently don't get the reliable secure benefits for which XLR contacts are so well known. The physical and electrical design is compromised resulting in potentially degraded sound quality. The root cause is the XLR connector is allowed to wiggle around and break or never even make good electrical contact. Beware!
The solution is to use balanced cables with female XLR connectors at the source end and TRS connectors at the amplifiers combo input end. This makes for a tight, reliable fit and overcomes the combo connectors compromised design limitations.
For unbalanced sources use TS connectors at the amplifiers combo input. Hope this helps solve a problem you weren't even aware of. It took me awhile...
1/4" TRS Phone Plugs and Jacks
TRS stands for Tip-Ring-Sleeve, the three connections available on a stereo 1/4" or balanced phone jack or plug. TRS jacks and plugs are used for balanced signals and stereo headphones.