I can leave this to you and Chad but to say that unless you’ve got a so-called “Moritz” high G#/A ring then you guys can probably open those tubes up a lot to help you (If you’ve got this extra key and the C was moved down-bore to accommodate then, well, based upon my experience, good luck:). I’ve had the B and C tubes of more than one of a few different manufacturer’s horns opened and/or flaired by household names of bassoon repair humans over the last 20 years. Plus, a couple of those horns have had the large and small vents of the Bb tonehole changed in this time as well. In all cases it was an improvement. Don’t forget a slightly-opened low Db key on both of the notes you mentioned, the Bb in particular – don’t leave home with Tschaik 4 or the Berceuse without it, IMHO. If you vent the notes, close the whisper key and you’ll have much better luck, again, IMO – with this, and with all my previous statements about this stuff, I defer to those who know more than me which is a whole bunch of you reading this!
We’re off subject of course, sorry, but keep the tubes open during drying with the same or similar drying pin (I form and dry on the same taper in most cases, though this is hella expensive – reed making is VERY expensive if you experiment) so I don’t deal with too many variables. This is very important because a LOT of cane will shrink like crazy during drying so you might as well form on the nail as well given the changes you’ll deal with later
The Rieger mandrel works great for many of the standard Rieger shapes, but, well, you need to experiment. Find out who designed X or Y shape and then find out what mandrel he or she used during the process as well as what gouge thickness, cane length, bevel, and wire/collar placement was used – in ADDITION to the basic thickness taper/dimension. This is critical. It is a part of his/her experimentation in developing the shape in question and he/she will take it for granted while you may be struggling like crazy. Change these aspects only AFTER determining your success with that which was a part of the design process. IMHO, which many may disagree with, but basically, as with all reed making,
To answer a couple things:
I play a Moosmann 222A
I usually form on a Rieger forming mandrel but let the tube dry on a nail (or might as well given the shape of my drying pins). I use a regular Rieger mandrel for tube and wire work but us a real short pin, no name mandrel for scraping.
My usual bassoon repair human is Chad Taylor, who posted in this thread a couple posts back. He knows my horn and might have something to say about this idea of my low tenor register. I will say that my Bb and C are fussy notes that really require a solid centering to get in tune, so it’s possible the pitch problems are as you hypothesize.