I found out that my amp is not as loud as it was in the past. Half year ago when I put a 1/4 of its' max volume I couldn't be at the same room. It was too loud. Now I can have 1/2 of its' max volume and it's similar. I can't be at the same room. But why now it's 1/2 and a half year ago it as 1/4? Is something wrong? And I noticed that the sound is a little bit different that it was in the past.

My amp: Marshall MG15CDR

Please help
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