Why must I use alkaline batteries (instead of rechargable) in my new guitar amp?

I bought a portable guitar amp that runs on batteries, but the manual says to use only alkaline batteries. Why would it matter? Why cant I use rechargable batteries?

Alkaline batteries (new) have a voltage output of @1.6vdc. Rechargable batteries, fully charged, rate at @1.2vdc. Your amp probably requires the higher voltage to operate properly.