I bought a used Marshall JCM 900 50W head a few weeks ago and have been recently experiencing every once and a while the volume will cut considerably and the note or chord being played sounds like a rattle-y buuzzing version of it's former self. Are the tubes in this bad? My friend said that he thinks they're the stock tubes that have been there since the amp's production (im a little hesitant to believe that when we found it was made in '95) I also picked up on that one of the tube retainers rattles when I pick the amp up, move it, etc...could that have anything to do with it?
Forgive them Father, they know not what they do.
Tube rattle will not cause volume drops

Tubes can last 20 minutes or 20 years (or longer)

Those tube retainers drive me insane so I removed mine

Try separating the cab from the head as far as possible (preferably with a speaker cable) and see if you can hear any rattle that way

I would seriously get a new matched set of power tubes anyway and get it biased.