You should definitely try a Tube preamp. I have N804's and it changed by system sound tremendously- really brought out some sweet bass. The mids and highs were always very good. I have an adcom 545II amp and it sounds great, so your Hafleer should be fine. I have a Cary SLP94 Pre, used under $1,000. I'm very happy with its sound!!
Good Luck!
Good Luck!