No problems with the weight on the sub ..But technically the sub will introduce vibrations to the amp which will in turn amplify these vibrations as distortion..
Ever seen those 300.00 amp stands for sale at some high ends store
..made specifically to keep out unwanted vibrations...( kinda like how a turntable will amplify vibrations) ..
Now, real world results ? ..I had to do that for awhile and didnt notice anything ..but it got in my mind, so I just had to get that amp off of that sub ( or else it would have drove me crazy
I would rather just tuck it away behind the sub ( if you have an auto on/off amp )