Hi, Maddie.
Both Babylock and Brother have at least one model that takes 90wt bobbin thread.
When these machines first came out, even some dealers were unaware the machines required a bobbin thread that was not the norm.
If it is difficult for you to find this weight of thread, ask your dealer to adjust the bobbin tension or replace the bobbin case with one that will accept 60wt bobbin thread.
Here are a couple sites that have 90 wt, but with shipping this is way too much (IMHO) for bobbin thread. http://www.moores-sew.com/servlet/the-701/The-Finishing-Touch-White/Detail or http://www.icanhelpsew.com/janome-bobbinthread.html
Not all letters sew as nice as others. If you have a software editing program, you may be able to add under stitching to better support the letters. Satin stitching needs parallel lines at the edge for the satin stitch to wrap around. You can find more on this in John Deer's book "Digitizing Made Easy".
In the meantime, I would try using the same color thread in the bobbin when doing the letters to see if it helps - you have extra shirts to experiment with.
Good luck.
Pattiann