Selmer S[80] C* are notorious for not being finished properly, I bought 2 in the last year one was barely playable and the other needed to be sent to a mouthpiece refacer so it would play, but the i could also be the fact that you haven't adjusted to the pressure of the new piece yet. I would recommend sticking with the 3's give it 2-6 weeks and see what happens. If you aren't that patient send to a mouthpiece refacer as their are tons of them that post on the forum, to name a few: Brian Powell, Eric Greiffenhagen, Keith "Mojo" Bradbury, Adam Niewood, Morgan Fry, Sakshama, Phil Engleman(who did a great job on the C* I sent him) and Johannes Gerber, Freddie Gregory and Aaron Drake etc... Also do a search on the forum for Selmer C* and see what comes up, go to the tone producing section and read what Phil Barone posted on tone producing.
And welcome to the SOTW community