In 2006, after I got back from Iraq I flew out to Alembic in CA. I had ordered an Entwistle "Exploiter" Series II replica two days before I left, and was there to finalize the final details of it.
As I sat with Susan Wickersham for this purpose, I asked her about John's Alembics. She told me that when they completed them and got ready to ship them, they were set with the action that he liked, which was, as others here have noted, was insanely low. She also told me about the time they'd completed a bass for John when the Who was on tour and in CA, and that they were going to deliver the bass to him there. They had the action set how he liked it, and no one at Alembic could play it without horrible buzzing, fret noise, etc. So, they go to see John at the gig, taking and employee with tools in case it needed adjustment.
They get there, give John the bass, and he plugs it in. I forget which tune he began to play, but he played it without any problems whatsoever. Clearly his light right hand technique made all the difference in the world with action that low.
As for my replica ... It is at the same time the same and different. My action isn't like John's, and there are some differences (nut width, pickup and control layout/placement, pan pot instead of selector switch, gold plated hardware, inlaid logo, sterling silver wire for the shaprs and flats, etc.). If you like John's Alembic tone, you can buy a cheaper Alembic and come relatively close. One thing about Alembics, though (I own three) is that it is very unforgiving of sloppy technique. Every mistake you make will be amplified, in spades. This being said, you will be rewarded with clear notes - everyone you play will be heard clearly. I've had other people tell me this when I play my Alembics.