I have a free bass Morino model that has its treble reeds in wax and its bass reeds on leather, and that has L on the treble in déclassement but just the bass octave in the bass with elongated reed chambers making for some approximation of a cassotto effect.
If I use LM on the bass side, the tone quality is very brassy and direct while it is quite more civilized on the treble side. I use that distinction in accordion orchestra, playing some parts with the left rather than the right hand.
So while I can second your observation to some degree with my instrument, I make use of it for either getting this fanfare-like sound with LM or a single-reed L registration that can hold its own against various treble registrations.
Check out the
rough sketch of BWV934 I recently posted for various combinations of free bass and treble registers. Yes, I'll follow up eventually with something more serious.