Hi Dave,
I think what you described is the case, as even with al valves pressed down there is almost no resistance. The instrument just plays extremely open and it will take a lot of practice to get back used to it. I actually played a Besson 2052 Prestige from 2012 and a Sovereign 967 from around 2000 (very late English production) before I got the Round Stamp and while it was getting repaired. Both are indeed more responsive and the prestige is particularly a very hard horn to play on, but the Sovereign could sing! For your interest, here is a side by side comparison with the Prestige and the Round Stamp.