Subwoofer makes a thumping noise when A50+ amp is powered on

I use a A50+ amp with Elac Debut 2.0 B6.2 speakers and an active JBL subwoofer. Most of the times the subwoofer makes a loud thumping noise when the amp is powered on. Can anyone tell me whether this is normal. This wasn’t heard when I was using the sub with Harman 5.1 amp.
The subwoofer is always in Auto mode i.e. it is always in standby and automatically starts working when it receives a LFE signal from the music that is played. But in the case of A50+ the subwoofer switches on from standby the moment the amp is powered on and before the music is played. Need a clarification on this also.