Some of you know that the "Symphony" back-bore ~and~ the "Orchestra Grand" back-bore will BOTH bolt up smoothly to EITHER of the Symphony or Orchestra Grand cups.
- The "Symphony" back-bore features a reverse taper in the throat area, which tends to offer low range facility a "boost" with many 4/4 and 5/4 size contrabass tubas.
- The "Orchestra Grand" back-bore features a conservative opening size, which offers some assistance in controlling/focusing the low range of many 6/4 size contrabass tubas.
There IS, however, a THIRD back-bore taper (not ordered by many...therefore we don't produce as many of them) which bolts up to BOTH the Symphony and Orchestra Grand cups, and is (well...) just a "regular" (medium large throat, classic tapers) back-bore. It's simply called "X" (and yes - just as with the other two - is available in standard, P (in-between), and euro exterior shank sizes)...
...so if you neither desire a back-bore which does what the Symphony back-bore does, and nor do you desire a back-bore which does what the Orchestra Grand back-bore does, perhaps you are one who might (simply) prefer a (though rarely sold) "regular" X back-bore.
I keep the "X" shanks in stock (here at blokeplace - Mid-South Music, Inc. in Tennessee)...If more are ordered (just fwiw...), we will produce more of them.