Never heard of people repairing tubes before but I hope it is possible for your sake!
My sophia mesh 300B tubes had loose bases and rattling sounds when u move it around. I was told I could glue the base onto the glass which I did and its stuck now. But the rattling noises still is there. They both work fine though.
That's all the experience I have had with loose base tubes ..
I do notice with my brown base u52 it doesn't light up very bright at all.. can only see a bit of light at the top of the filament. During daytime, I probably wouldn't be able to notice if it is lit at all. Have you tried touching it to see if the tube is warm at all?