Most all Holton 345 tubas that I've played (either length) seem to feature a right-up-to-pitch 5th partial open D/E, a sharp 6th partial F/G, and many feature a low 3rd partial F/G (which, sometimes, isn't flat, but is mistaken for flat because the SECOND partial Bb/C tends quite sharp).
This 345's tuning tendencies (again) are more like a Miraphone 186 or King 2341 (not-terribly-flat 5th partial, and other pitches hovering acceptably close on either side of the center).
I sorta wonder if the tendencies will remain as they are once the huge crunch is removed from the bottom bow.
What if (and joking...) I have to re-crunch it, and hide the crunch with the new bottom bow cap...??
Something that the BB-flat versions of the 345 tubas feature is "guts" in the sound, whereas (it just seems to me) the C versions of these - as well as of most other 6/4 C instruments) don't offer as much "guts" in the resonance (ie. more "hollow"-sounding...which only makes sense, whereby the same mouthpipe, capillary bore, and bell are mated up with a 2-feet-truncated bugle taper).
Many are fairly convinced that the Holton "copies" of Mr. Jacobs' York are actually just re-worked versions of previous Holton 6/4 models (yet with a York-looking valveset, and a smaller final bell diameter...(a larger 6/4 C instrument than the "good" CSO York).
To me, the Yamaha 6/4 (though most others didn't "vote" for it in some guy's comparison-of-several-6/4-C-tubas video) is the one York-alike that - at least, to me - really seems to be designed AS a C instrument...and it also seems to be a bit smaller than most of the rest. Additionally, the Yamaha (YCB-826S) offers intonation tendencies ("in C") very similar to those offered by this Holton BB-345.
The best of the Holton factory C 6/4 instruments that I've personally played is the one that belongs to Don Little. Its tuning gadget is pretty darn cleverly-devised, too.
IF (??) I were to ever again own a tuba of this specific style family (and I've "been though" several of them), it would - most likely - be a B-flat one, and would need to play as easily in tune as does this customer's instrument.
===========================
off-topic:
Even though it's got a couple of "it isn't finished being built" issues, I'm REALLY enjoying picking up my only-32-inches-tall COMPACT 19"-bell/smaller-bore Holton B-flat tuba, and playing on it (rehabilitating myself for some upcoming jobs - back from quite a few months of barely touching my face to tuba mouthpieces). By the time it's completed, I'm pretty sure I'll have already learned it's tendencies and "what it needs". It makes me chuckle when I rest it on its bell: The upper #4 slide (in it's "in-tune-for-low-F-pulled-out-an-inch" position) comes within only about two inches of hitting the floor...reminding me - a good bit - of this imagery:
