It is possible your amp is expecting pro audio levels on the XLr and therefore it is atenuated somewhat when you use XLR with a device that does not output pro audio (+4 dBU).
Roughly speaking, consumer uses cheaper components and runs at (10 dbV = 0.3 Volts) while pro uses stronger signals (+4dbU = 1.22 Volts). Pro gear can usually go up to about 12 to 15 volts while consumer gear runs no more than 2 volts.
Ii can't be sure - but this might be the reason and it therefore means there is nothing wrong with your gear.
Roughly speaking, consumer uses cheaper components and runs at (10 dbV = 0.3 Volts) while pro uses stronger signals (+4dbU = 1.22 Volts). Pro gear can usually go up to about 12 to 15 volts while consumer gear runs no more than 2 volts.
Ii can't be sure - but this might be the reason and it therefore means there is nothing wrong with your gear.