Theo basically has it right.
It is basically just substituting different tubes from the stock tubes that came with the unit. Typically this yields different sonic results depending on the tubes. Usually better results, but sometimes worse. (Typically, but not always, NOS (New Old Stock) tubes will yield better results than current production tubes, since they were manufactured during the peak days of tube use (i.e. in the '50s and '60s). And, as you can imagine, NOS tubes tend to be much more expensive than stock tubes, (and get only more expensive with time, since they are slowly but surely getting more rare, as the supply is used up).
To get an idea of the sonic differences between tubes, check out Joe's Tube Lore @ Audio Asylum, (to which I have provided a link below).
Joe's Tube Lore @ Audio Asylum
I hope that helps!
It is basically just substituting different tubes from the stock tubes that came with the unit. Typically this yields different sonic results depending on the tubes. Usually better results, but sometimes worse. (Typically, but not always, NOS (New Old Stock) tubes will yield better results than current production tubes, since they were manufactured during the peak days of tube use (i.e. in the '50s and '60s). And, as you can imagine, NOS tubes tend to be much more expensive than stock tubes, (and get only more expensive with time, since they are slowly but surely getting more rare, as the supply is used up).
To get an idea of the sonic differences between tubes, check out Joe's Tube Lore @ Audio Asylum, (to which I have provided a link below).
Joe's Tube Lore @ Audio Asylum
I hope that helps!