My guess is that an amp isn't going to help you if you place those speakers in between those walls. A space like that would most likely be an acoustical nightmare and would explain why the speakers sound better when they aren't in there. Getting a newer, more powerful amp would just drive more power into an acoustical nightmare, just making it a louder nightmare. What I would suggest would be to leave the speakers where they are if possible. Hope this helps.
Mike
PS. The reason you get the added bass in that area is because you are effectively putting the speakers in a box, making a larger speaker cabinet of sorts