Moving a sub into one of those wouldn't necessarily degrade the performance, but there are some things you need to keep in mind. How inert is the enclosure? If it is made from flimsy, light material, the sub will vibrate it and everything on it, and that will hurt the sound. If it is solid wood, it might be OK. You still might want a soft pad underneath the sub to damp its mechanical vibrations. Something else to think about is ventilation. If it is a tight space, there might not be enough ventilation for the amp's heat dispersion. That will shorten the life of the amp. Make sure whatever sub you put into that isn't a really tight fit.
