If you really want that 'tube' sound - Behringer makes the perfect product for you. It's called the Behringer T1953:
http://www.zzounds.com/item--BEHT1953
Place this between your solid state pre-amp and amplifiers. You do not want to use tube pre-amps or amplifiers with this unit, as this unit will add in all the 'sound' you want in a controlled manner. This unit lets you dial in the exact amount of tube 'sound' you desire, from none to heavy amounts.
-Chris