Higher sensitivity speakers. Will make the biggest difference, especially if your looking for a $1500 integrated to fix your problem. Your 400's are rated at 87db, which means they are probably along the lines of 85db. I've never heard your 400's but with those specs I would imagine you need to turn it up a little to hear all the details, dynamics, and bass on the same level. Lower the volume and you are missing a couple quality's of the loudly played music.
If what I'm describing is the issue you are having then I think you should look for higher sensitivity speakers. If what I described is not really a problem for you then spend your money elsewhere. Good luck