Question
leaves curl up, turn brown and dry out.  The whole bush(s) is becoming affected

Answer
Check the lower stem of the plant and look for splits or cracks in the bark.  If you see anything like that, it could be cold injury from last winter.  Altheas are notorious for this problem.  They will also just die off for no reason.  I have several and they just check out with no apparent cause.  I would expect your plant to totally die at this point.  I would make a cutting to replace the existing plant.  Sorry for the hopeful news.  Good Luck.