A very interesting idea, but not supported by my experience. I have played numerous concerts on 6/4 tuba with various ensembles and a number have been recorded with me having an opportunity to listen afterwards to how it sounded at the back of the hall.Ken Crawford wrote: You may not agree, and that is ok. (But I'm right) <---this is a 5/4 tuba btw.
In no cases would the 6/4 tuba not be heard - but had a real presence distinct from recordings where I have been playing smaller tuba. I have also listened to others bands with and without 6/4 tuba playing in the same acoustics. Inveriably the band with a 6/4 will sound better, with a real bottom to its tone.
If you don’t like 6/4’s then don’t play them (that’s fine), but don’t pretend they need the help of trombones to sound, as that is not supported by reality.