It's like getting a new amplifier!
I just popped a set of these in my '65 Fender Bassman and love the sound! While they may not give me higher power, there seems to be more headroom and punch; dynamics are bigger and better controlled; and the sound is very open and clear at every frequency with tons of very solid bottom-end. They also seem quite stable, well-matched, and the bias can be dialed-in perfectly. All of my 6L6s are getting tossed out and replaced with these!