Gear makers need to have consistency. Every unit they sell should sound the same. So they are going to use a tube that there is an ample supply available. Conrad Johnson does use good tubes and they make an effort to test and match them. There is nothing wrong with using the stock tubes. If you ruin your brand new $5,000 - $10,000 piece of gear fiddling inside the case, you can forget about the factory warranty.
The biggest variation in sound is with simple tube gear. There is not a lot of circuitry, so in many cases you are actually listening to the tube itself. In more complex gear, changing tubes is more of a tweek.
Changing tubes will definitely make it sound different. Better is in the ears of the beholder. Since there will be no agreement on which brand of NOS tube is better for any particular piece of gear, then it really comes down to personal taste.
Once I find a brand of tube I like for a particular piece of gear, I almost never change which tube I use after that. Some people like to tinker.
The biggest variation in sound is with simple tube gear. There is not a lot of circuitry, so in many cases you are actually listening to the tube itself. In more complex gear, changing tubes is more of a tweek.
Changing tubes will definitely make it sound different. Better is in the ears of the beholder. Since there will be no agreement on which brand of NOS tube is better for any particular piece of gear, then it really comes down to personal taste.
Once I find a brand of tube I like for a particular piece of gear, I almost never change which tube I use after that. Some people like to tinker.