Found out recently that mouthpieces really do affect intonation. If you browse the internet, you will get some general information that shallow mouthpieces make you sharper and deeper mouthpieces make you flatter. This is logical to me, as a shallow mouthpiece means that there is less room for the air to travel to get to the horn, i.e., the horn is essentially shorter and therefore, of a higher pitch. However, I think there is more to it than that. As I play around with various mouthpieces and the overtone series on the trombone, some mouthpieces are just a better match for me, intonation wise. This is not something that you think to try at the store when you are testing out the mouthpiece (at least I did not think to do it), i.e., use a tuner with the new mouthpiece (& horn) and see if you are playing any differently with the new equipment. I would recommend doing this, as I found that the mouthpiece that felt and sounded pleasing messed with my intonation. It is a lot of work to get your intonation back in line, so it is something to consider before making a major change in your equipment.
Dr. Michael Kearns
Musician, educator, husband, father, web designer ... my life is like a mosaic with each piece vying for my attention.