Why do my Amps do this?

I have a pair of 140 wpc tube amps. Last night when I shut them down (after 3 hours of listening), I noticed that when I flipped the left switch my left woofer moved out and then in. Same happened on the right side. So I waited a minute or two and powered them back up. Same thing--each woofer "took one breath." When I powered them up this morning (after sitting idol for 7 hours), this did not happen. But everything sounded fine. I never noticed this happening with my last amp (45 wpc). Does this indicate a power surge that I should be concerned about? Thanks.