If your current preamp is only flat sounding, I would keep it for now. Its very hard to find a good, inexpensive preamp in your price range; new or used. I would consider a passive solution but not active. If you go active, you are likely to get something else that sounds flat; or worse, you'll end up with something that is bright or harsh. The preamp is too just too important. The only cheap way around it is to not use one.
