I think it is very important that you come at least £300 below a custom job from the manufacture. I know if I were spending £1000 onn a new amp and I could customise my own for a few more hundred I would, personal opinion.
You need to come up with a all new feature that will make it unique. There are too many people using the SVT's and DB751 for you to compete in that market and people like to have the amp of their favourite bass player to try and get their tone in my opinion, so developing something like switchable wattage like has been said or a professional on board compressor that you can adjust attack release etc.