I am pretty familiar with the 188. To "
open up the low register", try going
smaller instead of going bigger and bigger with no end in sight,
unless you are over-blowing your current mouthpiece. Give the Yamaha Roger Bobo symphonic, or Yamaha Jim Self a try (alternatively, Miraphone or Schilke 69C4). The reason is because with these smaller (but not deep into F tuba mp territory) mouthpieces they are not like vacunm cleaners sucking the air out of you and making it hard to sustain phrases, or low range, like some huge mouthpieces would do to you. The rim diameter on the Bobo symphonic model and Jim Self model are big enough (bigger than the Schilke and Miraphone) to ensure richness in tone in the low register. Personally I use Bobo symphonic on my CC with no problem playing low and loud. Some may find the Bobo somewhat bright, edgy. The Self model is warmer overall.
If you want really huge
and dark sound in the low range, you'd pretty much have to go with one of these specialty mouthpieces like PT-50, PT-50+, PT-88, Parke Ofenloch. But as you probably already know, you
WILL be doing it at the expense of your higher register and overall efficiency. That said, every pro tuba player I know has a least a couple of mouthpieces in their arsenal. It just so happened that I have a great mouthpiece for that purpose, for your 188, for sale: Perantucci PT-88S. The S stands for small(er) shank. But it is really the same as the standard American shank, like that on the standard Conn Helleberg, which will fit the shank on your 188 perfectly. The standard PT shank will sit slightly further out. but it's not a really big deal. I played a 186 with a PT-48 with standard PT shank and noticed no problems. PM me if you are interested, I'm looking to unload them asap so the price is quite good and I'm offering packaged deals as well. I may even send you a few for you to try out if you're interested.
As to bowl or funnel, if you are trying to achieve the good old American sound, which 188 is NOT best known for, at least you could use a funnel-shaped mouthpiece. But the 188 came with a Miraphone C4 for a reason.
I have played many the mouthpieces currently available, and I feel extreme (in either direction) mouthpieces may hinder your development - in this stage, you have no objective need for them. A sensible middle-of-road mouthpiece is the Laskey 30G or 30H (make sure to get American shank, not European shank, for best fit).
Finally, the resistance level of 188 may offset the openness of some huge mouthpieces, and for want of a darker sound, I do feel you. But if you use a mouthpiece that's too big, the overall sound (maybe except low range) will be pinched - thus deter the very purpose of "openness in sound." After saying all that, the end product is the best (and only) indicator. If you sound good, then it's all good!
Sorry for mumbling, I must go to bed now.
