Lots on this already, but I'll chime in some too.
I play pretty hard, so buttery smooth is NOT equal to low for me. But regardless of your playing style, you do the same things to tweak your action for perfect playability.
Your ability to lower the action is bounded by your tolerance for fret buzz. When you pluck (or pick) the string, it vibrates perpendicularly from the direction you hit it. If you pluck exactly parallel to the face of the bass, then the string's motion is side to side across the neck. If you're not exactly parallel, then some amount of the string motion will be up-and-down relative to the fingerboard. That up-and-down motion over the frets is where fret buzz comes from. The harder you hit the strings, the more up-and-down motion you'll have. And the lower you set your action, the more likely that the up-and-down motion will hit the frets causing buzz.
This is why you generally dial in some relief or bow into the neck. If the neck/frets were perfectly flat, then it's highly likely to buzz since any up-and-down motion will clank on the frets. So, you dial a bit of relief in which in effect makes a little more space between the vibrating string and the fingerboard - voila, less buzz. For most quality basses (all Alembics qualify here), the truss rod(s) are key to getting the action the way you want. This lets you dial in the exact amount of relief that you need for your playing style, strings, and the climatic conditions you're playing in. Too much relief and you won't buzz, but the action will feel high and slow in the middle registers. Get it just right, and it'll feel low, fast, and buzz free. Learning to do the truss rod adjustment is one of the best things you can do as it will allow you to maintain optimal action regardless of the humidity or temperature.
Alembics are really superior instruments from a workmanship standpoint, so it's unlikely you will encounter some of the many neck problems that prevent the truss rod from tuning your action. With Alembics, you have an excellent chance of getting properly aged, straight neck wood, properly assembled, properly levelled, etc. With cheap instruments, the effect of the truss rod might not be even across the length of the neck - you won't get low action if your truss rod makes an S-bend in the neck short of a refret!
Straight from the factory, they would normally have done an excellent regular setup, but if you live in a place with radically different climate than when they did the setup, you'll need to at least tweak truss rods for best action.
If you really want to go super low, then you will need to make sure everything is super precise. When they build the instrument, the fingerboard is ground to precise flatness, then they insert the frets then grind the fret tops to precise flatness as well. If the fret tops aren't precisely flat (either due to a grinding error, fingerboard error, or a twist that developed after assembly), then the high fret will limit how low your action can be set. In this case, you should take your bass to a qualified luthier who lives in your geographic area. They'll do a fret grind or level which will bring everything into alignment. You should pick somebody good who lives near you so they'll be doing the setup in the same humidity and conditions that you'll be playing in. Then your setup should be optimal again.
Depending on your playing style, you will also experience fret wear. When that happens, you'll be buzzing again, since there are low and high points over the various frets. Usually fret levelling will fix this, but eventually, you'll need new frets. When doing a fret job, they'll pull all the frets, sand the fingerboard flat, refret and level the frets. Alembic uses great materials, but hold on to your hat if you need a fret job on your Series II with LEDs - you won't be a happy guy.
Once you get the frets level and relief set properly, you'll be in business. Given good construction (e.g., working truss rod), this is all that it takes to get a perfect pro setup. Most players are keyed into all this stuff, and it's just easier to try a lot of instruments until you find one that happens to be right for your style and location. But with a little work, any of them could be perfectly tailored for you.
Just a couple of other notes... Somebody asked about stiffer necks. That's isn't required for good action, but makes it a lot easier to maintain. Even small changes in humidity and temperature can effect your relief - since the wood is a natural material, you won't know how subject it is until the neck is built. Adding laminations or stiffners can help resist some of the environmental stuff and make it easier to maintain. A graphite neck (a la Modulus) is pretty much impervious to regular environmental conditions, so once you get it set up, it generally will stay that way. The caveat there is that the Modulus neck didn't have a truss rod until recently which means that once you built the neck, the relief wasn't going to be easy to change without grinding it into the frets. For most people, I think that was undesireable, which is part of the reason Alembic doesn't offer them anymore. I speak from experience here as the owner of a graphite Series II. The action from the day I picked it up was always buzzy due to lack of relief during construction and it's so stiff that you could tune it up a third with no visible change in the relief. When I got it I wasn't as sensitive to this, but over the years found I wasn't playing it much because it wasn't that comfortable to play. I recently had this cured by a super-duper fret grind that was done to match the relief to my main bass. The luthier that did this grind is the custom shop builder at Modulus, but with the LEDs, he told me that he wouldn't do it if he had to actually refret - too risky. It's great now though.
Somebody also asked about the importance of a flat fingerboard to low action. This shouldn't be an issue on basses, but flatter fingerboards are pretty common on shred guitars. It's for a totally different reason though. The curvature of the fingerboard is there to make it more comfortable for you to play, especially when playing barre chords on guitar. More curve makes it easier for you to barre successfully. But when you bend the high-E string across the neck, that curve will cause fret buzz as the string crosses over the center of the neck (think about it for a while and it'll make sense). So a lot of technical players went to a flatter neck to avoid this problem. Of course, it's much harder to barre then, but this doesn't seem to be much of a problem for Steve Vai, Joe Satriani, or Buckethead. Unless you bend sideways across the neck of your bass a lot, it won't matter.
There was also a question about small frets. This is also mostly a guitar thing, since your fingertips actually touch the fingerboard on the thinner strings. A taller, thin fret is a different feel. Unlikely you press your string down that hard on bass!
David Fung
