OK. But still a very simple troubleshooting step to hook up your 55 to the Panny plasma. Even though your Tosh HD-DVD player worked with the Mits, for some reason it may not be compatible with the 55, or possibly other mfgr BD players. The Mits may require a fw update, if there any available that is.
If the 55 doesn't work with your plasma, you know something is wrong with it. And it's probably not something corrected by firmware. If the 55 does work, well I guess you'd have to give Mits a call. Good luck.
Btw, I'm not familiar with the Mits, are there any other settings that may need to be changed? Oh, one other thing, have you tried a different set of component cables?