If I had to make a list on what makes the BIGGEST change in the Bassman RI amp it would be the following (Greatest to Least):
 
1. Rectifier Tube Type
2. Preamp Tube Type
3. Speaker Type (and Age)
4. Power Tube Type
5. Component Composition
6. Cabinet Covering & Construction
7. Cloth wiring, 2 conductor power cords, and other assorted mojo
 

With regard to the statement that all early RI cabs are built badly... I'd have to disagree there too. There's nothing inherently wrong with those cabinets as long as they aren't really abused to the point of losening the joints. And if one does happen to have a buzz, it's usually not rocket science to pull it apart and find the offending part causing the problem. If it's actually the cabinet joints themselves, a quick shot of wood glue will tighten it right up. But more often than not, it's the baffle being incorrectly seated.
 
Just my .02 ...