In reply to:
As long as the internal and external dimensions stay the same, it should not change the sound of the speaker.
I am actually toying with the idea of getting my speakers refinished.
Actually it can change the sound of the speakers; nothing too dramatic of course. MDF, unlike most other speaker woods breathes a tiny bit. The type of finish can actually alter this and thus alter how it resonates and howthe overall balance will be; but I highly doubt it will be significant. It will make a difference form unfinished to finished wood, however, I'd think.
Always nice to have one's opinions backed up by pros. 
