Although my F tuba is not without its quircks, the value of the quint valve, particularly of a considerably larger bore, cannot be underestimated.
My first F tuba was a 5 valve with the standard flat wholestep.
The current horn with the flat halfstep valve and the large bore quint valve is much more versatile for low register fingerings, not to mention the option of being able to play low F (and E and Eb) with valves instead of as growly pedal tones. I would have a hard time going back to a 5 valve horn, or even a traditional 6 valve F.
Walter Hilgers had an identical F tuba early into his career, and he incorporated the quint valve into many of the Meinl Weston tubas he helped design.
The picture of the back of the horn clearly shows the quint valve past the main tuning slide. All of the tubing in back is the 4th and quint valve wraps.
