Change The Tube

Hi 3 month's ago I Bought Avantone CV12 i got it used

and now it's sounds not loud like before i mean it's loud but not like before i think the gain got down a little bit also when i turn up the gain

i get a little bit distortion it's like the sound dying

it's my first tube mic i don't know much about tubes

i don't know if is it the tube or the cable xlr 7 pain or not

Also what's the best Tube for avantone cv12

Thank You