Something to look out for if not already mentioned is that you need to be careful with inexpensive preamp claiming to be balanced. As Rower mentioned, many companies will install an XLR connector on the back of their unit an say it has "Balanced Connections" when in fact, it is simply a single ended input through an XLR connector to accomodate those with balanced cables. It is no different than a single ended RCA connection.
If you find a preamp you are interested in that claims to have balanced connections (regardless of price), be sure to verify exactly what type of connections they are.
If you find a preamp you are interested in that claims to have balanced connections (regardless of price), be sure to verify exactly what type of connections they are.