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Something may or may not be wrong with this picture. First, try it up an octave. Play open 2d ledger line C - bottom line G - 2nd space C to get your bearings. Then play bottom line G with 4 down. It should be the same as open. Then play 1st valve bottom of staff F. Then play bottom of the staff F T-4. It should be the same note. If it is not, then I'd look at mouthpiece, mouth cavity and physical intonation issues. Sometimes, particularly if you're playing or practicing in a small place, low notes can "fly off" and not intonate pitch properly, and you have to back up, think the big open throat and relaxed jaw, refocus a little softer but still good support and air flow, and get used to the intonation characteristics of your tuba in a different setting.

A lot of guys tune their thumb valve on the F this way an octave up, then pull just a little so it doesn't sound sharp on the lower octave. Others tune it completely differently. YMMV.

I'm curious. What does your 5th valve do by itself? Is it like a flat 1st valve or is it like 2nd & 3rd? If it's like 2nd & 3rd you don't use it with the 4th valve to play a low F. The MW-32 that I have has the 2 & 3 5th valve and does produce a false-tone G with 4 & 5. Most MW 32s that I've seen have the 5th valve wrapped so you can set it up as a flat 1st valve OR a 23 combination. If that's the way yours is, and you want to use it for low F you need to switch the slides.

Just to add to the confusion:

My MW Model 32 has a fifth valve slide that has a long adjustment, from a sharp 1st valve tuning to a flat 1st valve tuning (which is how I have it set). Valves 4 & 5 give low F. Valves 2, 3 & 5 give low F# and the C# below the staff.

This is the first I've heard of a 23 combination tuning for the Model 32's 5th valve. However, I'm not surprised.

By the way, the Model 32 has excellent "false" tones. That can confuse you if you don't know your way around the tuba.

Cheers,
Allen