I would tend to agree with other people who have said it seems like there's a problem with your signal chain. The Lexicon has pre-amps, and I wouldn't think you would need another gain stage if everything is working properly. Changing to a condensor, or a tube pre, or a new A/D should really have nothing to do with your issue. Make sure you have your levels up at every stage (hardware and software). Try a different cable, or mic. There could be a problem there.
Just to clarify, you are only having an issue with the input level being too low. You're not even dealing with output levels yet correct? What software are you using? (just curious)