"I just bought my first tube amp and I've been reading up on the process of changing tubes. Everyone says that rebiasing an amp is important for the tubes and the sound, but is rebiasing possible/necessary on all tube amps? There's no bias voltage adjustment that is readily visible on the my amp, so does this mean rebiasing requires opening the amp and is probably a better job left to the techs?"

Certainly changing to a different type of tube requires rebiasing. Replacing old tubes with the same brand and type may not require it, but it's still a good idea. In fact periodic tube rebiasing can help lengthen the life of your tubes and help your amp maintain a more consistent sound. All tubes have a bias voltage on them, and in order for the amp/tubes to be properly calibrated there has to be some way to adjust this voltage. The adjustment is generally found somewhere on the innards of the amp so if you aren't the techy type this is probably best left to a good technician. (WARNING: There are extremely dangerous voltages available on the inside of many tube amps. Don't poke around in there unless you know what you are doing.) Some players will tweak tube bias to attain a sound closer to their ideal, even though the settings they use may not be recommended or ideal for the amp and/or tubes. This is part of the art of being a good amp technician.