| The PRO version has separate gain and master volume control, so you can crank the gain to add grit. And the more gain you set - the louder the amp is.
Also there are more frequencies on graphic EQ (as far as I remember) and the poweramp is tube driven with gain control - that can be dialed full open to get the sound quite similar to your III (more "open") or set lower to get more compressed, dirtier sound.
The PRO version has also a "tuner out" output.
Tonewise they are pretty much the same. Some people say that older III are better, some that PRO is superior. I think the best way is to just use our own ears to judge.
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