Different tubes sound different and also distort different. ie; Groove tubes number system rates tubes on how early they begin to crack up. Same tubes, same brand, different breakup characteristics. Different plate voltages will also effect tube performance. Not just current and watt rating. If you want a "dirty" tube, pick a lower wattage tube that is not known for it's clean headroom. If you want a "clean" tube, pick one that is of a higher wattage rating and is known for it's louder, cleaner sound. Guitar amps tend to run the tubes a little on the warm side. You could probably bias a little cooler and expect an improvement in headroom much of the time.