Plougot, yes you are absolutely right. My formulation there is wrong. I meant it as you specified. In it's high register, a bassoon at a cc7 of 22 should sound at, say, 30 db. But in the lowest register, again at a cc7 of 22, it should sound at maybe 40 db.
Not to derail too much from the topic, but one remark on the Tchaikovsky: here's a live (!) recording with the RCO (Christian Thielemann conducting) where this passage works flawlessly with a bassoon!
Tchaikovsky 6. RCO
Admittedly, that's a rare case and it's very risky, but it can work. Especially after the importance of the bassoon at the very beginning of the exposition, I really think there's a structural and poetical logic behind Tchaikovsky's instrumentation there (and not a mistake). And that's completely lost when the passage is played on a bass clarinet.