Mesa Boogie Express 5:25 buzzing sound

Hello, i have a problem with my Mesa and was hoping somebody would know what could be wrong. I bought a Mesa Boogie 5:25 head and some random Marshall 2x12 cab in July and i quickly noticed quite an annoying buzz sound coming from the left speaker when i turned the gain knob to further than 12(I hear it more when playing high notes but it's always there).

I tried different guitars and cables, still the same problem. I assumed the Marshall cab was just plain bad so i traded it for a Harley Benton 2x12 with celestion vintage 30s, but then when i plugged it in i still hear that annoying buzz.

Could it be the tubes going bad or something else with the amp? Help!

Thanks in advance