More oomph (technical term) for one thing. I've had 845 SET monoblocks in my system for 8 years and when I hear a 300B amp here, it lacks the authority to move me. Which of course depends to a large extent on the speakers' sensitivity, and maybe yours, like mine, need the extra power that 845s provide.
Also, 845s seem to have a different quality of sound, hard for me to describe, than 300Bs. Certainly each has its adherents, and you might well end up liking your 300Bs better.
Finally, it's a question of WHICH 845s. The various 845 tubes in current production do sound different, and unfortunately I've found that the Shuguang Pavanes -- which cost twice as much as the more common 845A and 845B versions -- float my boat. I don't even want to hear the NOS stuff.
Also, 845s seem to have a different quality of sound, hard for me to describe, than 300Bs. Certainly each has its adherents, and you might well end up liking your 300Bs better.
Finally, it's a question of WHICH 845s. The various 845 tubes in current production do sound different, and unfortunately I've found that the Shuguang Pavanes -- which cost twice as much as the more common 845A and 845B versions -- float my boat. I don't even want to hear the NOS stuff.