i would second the rogue 66 pre as an intro to tubes. i believe there is one posted today on audiogon for $700 used, well worth it imo. i have a 99 mag preamp, different animal, but i think you'll find what you are looking for in a used 66 pre. also check out quicksilver's linestages, i've seen a few of their tube preamps going for under $500. i can't say i've listened myself, but others i trust suggested i listen when i was in your shoes. maybe check out vac, though unless you find a good deal on a used unit, i think this wil be outside your budget. i think there is a vac tube pre, their basic pre, nothing fancy, listed in the realm of your budget now on audiogon.
cheers
cheers