Well, I can assure you that music prx, or any other prx's will not work in the game folder....
put plugins in the SEPLUGINS folder

Next make sure you are using homebrew compatible with your fw
like, you might be downloading homebrew designed for later cfw or like 3.80CFW ect...

most should still work though.

If you can, post the homebrew you are tying to run
If I have the same homebrew you have I can zip it up for you, cause I am also running 5.00M33-6 and I have over 2GB of homebrew crap fully working, including that vista portal thingy (dunno why I downloaded that :\ )