If the high action is caused by the neck having too much relief (bow) then a simple truss rod adjustment will lower the action. Once a bass is set up properly for the way you play it, you generally don't need to adjust the saddles, only the truss rod. This is because the neck moves due to humidity and temperature changes.
However, if you torque the truss rod to lower action when the truss rod isn't the problem, you can cause a LOT of damage. ABG's are really big acoustic flat-top guitars, with little relationship to electric basses. Pay close attention to 202dY's post!!!
Think of an ABG as an air pump. The strings pull on the top of the instrument and cause the top to move as the strings are plucked. The bridge and saddle have to have enough tension on them to make that top move so it will pump air out of the soundhole. The heavier the strings you can play on it the better the acoustic sound you'll get. You also need a good breakover angle on the string where it crosses the saddle so that it will cause the top to vibrate. If you lower the saddle too much it WILL reduce the acoustic volume you get. And because almost every ABG made these days is already a horrible acoustic instrument, you don't want to make it any worse.
If the instrument has the typical piezo pickup under the saddle, it's vital that you know what you're doing when you sand the saddle. The bottom of the saddle must be perfectly flat and smooth so it will contact the pickup element correctly. In many cases, the saddle can't go down low enough to get a close "electric" style action without compromising the tone. In those cases you need to either have the whole bridge (the large wooden part that's glued to the top and into which the saddle is set) replaced or lowered (which again has problems) or the neck reset. Both are major surgery.
They have non-adjustable saddles because the hardware needed to make the saddles adjustable will severely impede the acoustic sound of the instrument. Gibson and Yamaha both made some flat-top guitars with adjustable saddles, and they sounded terrible. All that metal, plastic, and those connections kept the strings from transferring vibrations to the top of the guitar.
My advice to folks who don't know or understand acoustic instruments is to not buy it if the action is too high for you out of the box. But then I also strongly feel that unless it's a Guild B-50, Guild B-30, Ernie Ball Earthwood, or maybe a Tacoma Thunderchief, it's NOT a real acoustic instrument and it won't work as an acoustic bass for playing with other people.
John
