KFS-
Alembic will tell you that the instruments they make today are superior to the ones they made 20+ years ago, and I believe it from a finish and electronics perspective. As a player's bass, that 70's Series 1 could run you another $1500 on an electronics upgrade if you want quiet. Since $3000 is not an unheard of price for a recent model used Series 1, why risk the extra big ticket service bill if you're just looking to sound good?
I believe that buying an old one means you're collecting, or that you see something special in a particular instrument. If the instrument has a history, the price can go up, as it could if being sold by a known person. For example, if Dela was offering a bass, or if you want to sell that SSB of yours, I think you might see some decent offers.
There just isn't the same kind of rabid collectors market for Alembics as there are for some other brands. Don't ask me why, then I'd have to explain why someone would pay $10K for a late 80s PRS, and I don't think I could do that with a straight face.
PRS basses don't command the same kind of money as the guitars, either. I saw a fabulous players' version with a great top and birds (and serious buckle rash) going for under $1800 recently. If it were a guitar, it would probably have been at least $3K if not more. I can tell you I played a recent PRS bass, and there is absolutely nothing special about those. There's one that's been a few turns around eBay with a Buy Now price under $1K and they can't find a taker. The old ones were definitely superior, and I might have jumped that one if I weren't an Alembic biggot.
Last but not least, don't forget that eBay caters to the last-minute bidder. That bass could still top $3K if there aer a couple people who really want it. Just like anything else in the world, it takes two...