The flatness spec is for EQ accuracy. With tube variations it's hard to keep it down to +/-0.25dB, so I spec it a little wider. It changes up to 1dB simply by putting in used tubes.
For -3dB point, I usually forget to bypass some test equipment which rolls off at about 32kHz. It's a buffered and isolated iRIAA circuit. Probably forgot to use a straight iRIAA to test bandwidth on the CORNET. Maybe I'll do that in a couple of days. The TRUMPET ran out to 450kHz -3dB. And that's with wimpy 12AX7s.
"Airiness" is something that occurs between 10k and 30k. Usually it is caused by an overemphasis (non-flat EQ) such as an over aggressive tweeter. Extending well beyond 20k leads to a "realism". Often it manifests as a feeling or emotional perception, rather than a blatant audible response. So yeah, without bandwidth, you can't get either the "air" or the "realism". It's really wierd, I think, in that the wider the response, the less harsh the treble. But that's just me.
jh
