I used to wonder the same thing, even after years of listening to old Dead tapes where Phil's jamming away on some of their finest early creations. Even after getting an Orion which I adore and play all the time, best recording bass I ever had.
Then I finally got a Series bass. I wasn't CLOSE to prepared for the sound difference. What struck me most, almost literally, was the amount of clean, pure, low FUNDAMENTAL that came through. Like a steel beam to the head, only much nicer of course. SLAM. Some attribute that to the chambered body and neck-thru, but I think the electronics contribute a lot there also.
I'd seen the warnings here that the earlier basses might need an upgrade to avoid noise, but the original electronics in my 78 S1 are dead quiet. Haven't seen the numbers, but it's gotta be at least -90 db.
If anything threatens that, a few quick tweaks to the screws on the control cavity plate bring it back to a super-low noise floor in less than a minute.
The only real issue is the weight, and awkwardness from the body meeting the neck way up at the 24th fret. Small complaints compared to the upside. And with a flame myrtle top under a beautifully-aged finish which makes it look amber, she's drop dead gorgeous.
OK, time to go home and play it!
