Obviously, the Sig is a superb bass, compared to almost any other bass. It's an ALEMBIC, so how can you go wrong with a bass like that? The craftmanship of my Sig is immaculate and the sound is fantastic. Not to mention the beautiful dark cocobolo.
However, I think the Series bass is definately the best bass I've ever played. It is for a reason that Series basses are the top range of Alembic for 30 years now. It looks, plays and feels wonderfully and the sound...man, the sound...I just can't get enough of it!
To my taste the Series' sound is beyond belief and clearly superior to the Sig, in terms of glass-shattering power and overall tone. I love the stereo capability of the Series and the external power supply; no more worries about a battery running empty. Furthermore, surprisingly the Series weighs less than the Sig, so it is really more comfortable to play.
Duncan