Ampeg SVT VR + 610 HLF Problem

I recently bought an SVT VR to use with my 610 HLF cab. Right from the first use, I hear a distorted BLAAT sound whenever I hit the strings hard and let it ring, or when I turn the amp to above 9 o clock. My ears and my eyes seems to think its the top left speaker in the cabinet, but I cant be sure if it's a problem with the amp, the cabinet, or the instrument (american standard p bass).

The cab is a recent acquisition as well. However, I have used a friend's Acoustic 300W amp with the cabinet for some shows a few months ago with no problems.

Anyone have some insight for what it may be and what the best way to identify it is?

 

Update: it was the tubes. Got them replaced and it sounds like a dream.