I would suggest that the PT-89 is quite likely WAY too big for you as a tuba mouthpiece -- at least at this point in your tuba-playing development. I had a PT-89 for several years and played on it solidly for a couple of years when I had my Cerveny BBb horn. While it provided me with a very nice "gravitas" and tone in the contra-bass register, I ultimately decide that it was just too big for me. Too hard to control in the middle and high registers. Many tuba players use mouthpieces that are over-sized for them because they think "bigger is better" or because it makes the low range easier for them (as many euphonium players use small mouthpieces because it makes the high range easier for them). I feel that the standard description of the PT-89 is quite deceptive since the mouthpiece "feels" larger than it appears in the description and it has a BIG throat/backbore. The inner rim is also quite rounded, and this has the effect of making it feel and act bigger than its nominal 32 mm diameter. (Mouthpiece diameter is a kind of funky measurement since it depends on exactly at what spot down the rim you do the measurement. Particularly on a fairly rounded inner rim, a slight change in where the measurement is taken can have a significantly different reading.)
Here's another question that can provide a clue that the mouthpiece isn't a good one for you: How well does it play in tune across the normal easy tuba range -- say from the Bb below the staff to the Bb at the top of the staff. Are you having to work at keeping a bunch of notes in tune? Or once you tune the horn, is the intonation across that range pretty good? Does it "slot" well? Or do you have to kind of "hunt" for the note you're trying to play. Problems like this can just be a consequence of inexperience, but they can also be a direct result of a mouthpiece that's just too big.
See if you can find something like a Bach 25 tuba mouthpiece (not a 24AW please!!), or a Kelly 25, or a Schilke 66, and see if that makes a difference.
Finally, just because you have been playing euphonium doesn't mean that you know how to deal with the tuba. I my embouchure on each of those instruments to be quite different, for example. It's really not just a big euphonium. There's a lot more room in a tuba mouthpiece, and my embouchure changes a lot as I'm playing than when I'm playing euphonium. So your mid-range tuba embouchure may be different from your low (contra-bass) tuba embouchure.
However, I'd strongly recommend that you at least find one of the mouthpieces I mentioned and see what difference that makes for you.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)