However, a well designed 300B should sound different than a well designed KT88.Well, triodes do sound better than pentodes, plain and simple- at the same power levels. KT88s are capable of a bit more power than 300bs. Most people run 300bs single-ended while most KT88s are run push-pull. There is a noticeable difference on that account alone.
My preference for power tubes is also the 6AS7G; it allows me to make a triode power amp of high power and low enough output impedance that I don't need an output transformer- making the amp more transparent and wider bandwidth on that account. Due to the fact the amps don't use a transformer, they are more detailed and transparent than SETs (price no object) and I can build them at lower power levels where they work just fine with horns. But the biggest amp I've made makes over 500 watts. Cake and eat it too sort of thing.