When do you know when your tubes are going bad

I have a Music Reference RM-9 110W Dual Amplifier. It has been sitting in a box for a few years and dug it out tonight. My left channel is only running at about 10% as my right. I balanced the driver and tried to even out the bias but I am still getting a pulsation on the left with the bias maxed out. Should I change out my input tubes, output tubes, or put it back in th box? I really don't know much about tuning the amp but I know it was working fine when I boxed it. Any help would be appreciated.