Do you find battery supplies inferior to well designed power supplies?
I have to ask, because every single battery powered unit or battery supply I have ever tried sounded great for the first few tracks, then focus gets blurry and things sound lost.
Batteries that have a amperage capabilities should always be better than any made up mains powered supply.
There is only one reason I can think that yours did what you said, is if their were no voltage regulators after the batteries and the after a few hours the voltage dropped and as a result so did the circuit ideal operating parameters.
Cheers George