From your description (" very thin and cold midrange. I am looking for something fuller and warmer") you're the perfect candidate for a tubed preamp. In my experience, that's what tubes do: make most systems warmer and give that subtle midrange bloom. Some of the regular folks around here don't think words like "warm" or "bloom" should apply to good audiophile sound (they prefer accuracy over all else) but there are others who disagree and enjoy these qualities - who knows what accurate is on most recordings anyway?
There are plenty of good tubed preamps out there that don't sacrifice detailed resolution to achieve this (deHavilland, CJ, ARC, Rogue,BAT, Modwright, VTL, Cary, are just a few). I'd see if your local dealers would let you audition some at home or search the archived discussions here - just type in Bryston 3bst.