Im back and again ready to throw in my hat on the subject.
IS THERE A DIFFERENCE BETWEEN GAIN ON AN AMP AND A "DISTORTION PEDAL"
They are essentially the same and largely built with the same components and have the same effect.
Some pedals use a tube such as the 12ax7 that you will find in most all tube amps gain stage, most pedals use no tubes relying on hard electronics alone, some amps have no tubes (Solid State), (Mosfet) in their preamplifiers gain stage.
Distortion pedals are the preamp gain stage recreated with generally more control over the gain with level, tone and gain and luckily are made to create different qualities of gain that appeal differently to our ears. The gain stage on the amp is fixed and what you got cant be as easily switched out as a pedal.
There is a slight difference order that the pedal is processed in front of the amp as opposed to the gain stage in your preamp.
If you value your hearing and want the amp to break up without a pedal at tolerable levels there is no replacement for a tube preamp with gain, it is what the pedals are attempting to create. And as an added bonus you can still put your tubescreamer or whatever in front of the amp to push it even harder when you need it.
Not easy to answer because to many this is a subject of what is appealing to the player and much simpler to address if we were comparing a specific amp to a specific pedal due to all the differences between equipment. I personally play through three amps all that react very differently in how they are designed to operate and im still experimenting in search of the ever elusive tone, but im getting close.
DISCLAIMER: THE OPINIONS REPRESENTED HERE SHOULD BE TAKEN WITH A GRAIN OF SALT AS THE AUTHOR HAS BEEN REPUTED AS BEING FULL OF CRAP AND WOULD SELL HIS SOUL FOR A GOOD BOTTLE OF TEQUILA.