I know that you can get power tubes rated soft, hard, etc depending on how early they start to break up when you push them. When I replaced the power tubes in my Egnater Tweaker recently, I just went with a medium set because I wasn't exactly sure how the different rated sets would affect things. At the volume levels I normally play at, I'm getting just a touch of power tube breakup that adds a nice thick punch to my overall sound, but if I could get a little more it would take my amp over the top and sound exactly like the tone in my head. My question is how much of an effect would using a "softer" set of power tubes have on the amount of breakup I get at a particular volume? Is there a big difference between the soft and medium ratings or would I likely be wasting money?
it depends between the tube maker as well. you will notice a difference but i dont think it will be as astronomical as you think. you can also bias your amp 'hot' or 'cold' but that can have a difference on the length of tube life

sorry for not having a clear cut answer
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The numbering system isn't really about a tube being hard or soft, or breaking up earlier or later. It is about bias. In a given amplifier circuit, every tube reaches a particular current draw at a particular bias voltage. So, at a given bias voltage, every tube will draw a particular amount of current. A tube that draws relatively more current is given a low number. A tube that draws relatively less current at the same bias voltage is given a high number.

The kicker is that if you set the bias when you replace your tubes, you negate the whole sillly numbering system idea!
Bruce Clement
BC Audio Hand Crafted Performance
I should add that the benefit of the numbering system is that once you have your amp biased correctly, you can buy more tubes with the same rating, and be fairly certain that the bias will be close to what it should be without rebiasing. This is helpful if you have to pay a tech to bias your amp.
Bruce Clement
BC Audio Hand Crafted Performance