Jebsmith73: The MS-4's are also rear ported.
I have the MS4's in a 17d 20w room with 14' ceilings. Realistically though, the speakers are at the end of the long wall (the 20' wall, see my system for more info) and are about 6 feet apart and 1.5' off of bookcases that are 15" deep. I sit about 8 feet away (the setup is because they are in a living room that I, sigh, share with my wife). Bass is NOT a problem. I've heard the MS4's in a larger dedicated room and found them lovely at all levels, with especially pleasant LF extension. I think the MS5's would be too much speaker for your room. If you have a dedicated room than you're in good shape to be able to tune in when you find a speaker you like, but I wouldn't assume that just because you're using two 6.5" drivers now that another speaker with this setup will sound the same.