I have a GB spitfire and can honestly say its the best all round bass I have ever owned! I know Bernie personally and he's a great guy-always got time for a chat and will sort any problems that you may have (although I've never had any!) Sound-wise, I have an old 63 Jazz and would say that the sound is quite a bit different, more linear, although the pick-ups in the spitfire are so powerful you can get pretty much any sound you want. Bernie can also change any aspect of the pre-amp/ pick up set up (ie frequency sweeps etc) that you want as the pre-amps have been designed 'in house' and not bought in from any other manufacturer. Word of warning though-most spitfires have 18v circuitry so you need an amp with alot of balls to handle it, and also to get the best sound out of the bass!
Hope this helps