How to balance between Bass and Contrabass?

[Rick Denney]

...But the 621 has a ceiling above which it will not go...

I've heard/read this adage/truism often enough.
I'm skeptical of it's veracity.
Consider the two examples of users you cited in your post.
With no consideration of noteworthiness, does either of them use theirs to perform with a *large symphony orchestra?

bloke "again, pretty sure a 621-F will do (when properly asked...though, perhaps not with the most complex resonance) most anything that's needed of it - wherever it's needed"
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*the only point here being that the 621-F is rarely put to (what many consider to be) the ultimate test...so - without being tested - how can it pass?

...and no, I do not currently have one of these in my possession...so no "carnival barking" going on here

1. Yes. From a performer in a "large symphony orchestra".

2. Yes. From me. When I played stuff that should be on bass tuba in the Loudoun Symphonic Winds, the 621 sound is mostly lost. The B&S sound is not.

For small chamber groups, it's not an issue.

Rick "who has conducted the test" Denney

[Rick Denney]

A friend of mine (plays in a full-time orch. in your state) uses a 621-F...

...puts out plenty of sound.

' has had it for not-that-long...' has had to learn to back off a bit from what "feels" like "fortissimo".

Good for him.

Rick "whose experiences are his own" Denney