1) The preamp tubes are smaller than the power tubes, and are usually numbered either 12AX7, 12AU7 , or 12AT7. These are the initial tubes that amplify the signal of your instrument, and provide the overall timbre of the sound that is later further amplified by the power tubes. The 12AX7's will give more power and distortion than the cleaner, audiophilic-like, sounding 12AU7's.
Read more on twin triode amplification factors.

Preamp tubes tend to be "microphonic", which means that they can mechanically pick up and transmit external noises. Check them with a wooden chopstick !!

Preamp tubes are used as the drivers for the power tubes. The preamp tube closest to the power tubes, i.e., driver tube, typically drives the power tubes, and changes to this tube generally have the most impact on the amplifier output.

Different combinations of tubes can be used to produce different sounds, and give more or less headroom to feedback. The best advice is, as always, to try it out yourself and decide what you like for the music you want to play.
Check here for Jeff Harpdog Hale great experiments with pre-amp tube swapping on Bassman RI.

2) The power tubes are the larger ones inside your amp, and do the job of converting the signal from the preamp tubes into the power needed to drive your speakers. There are several types of power tubes available, each offering different performance characteristics. For example, EL-34 power tubes tend to distort quickly and easily and give a creamier sound with a looser low end. These tubes are mainly manufactured in Europe. 6550's are manufactured in the USA, and they stay cleaner even up to full power. When they do distort, it's a "heavy metal" type of dirty distortion, with lots of power. 6L6 tubes have a very good dynamic range and give the traditional "American Rock" sounds, while 6V6 tubes produce a creamy sound with nicer distortion. 5881's can also be described as having a softer less brittle sound than 6L6's, which is normally considered better for harp.

Here's a great tip that may even save you money.  Used power tubes tend to lose harshness and have decreased treble response, which is great for harp!  You may be able to get them cheap or free from guitar players or music stores that have replaced power tubes that have become too weak for guitar player's taste.

3) Amps also have rectifier tubes, or solid state rectifier as in the stock Fender Bassman Reissue (RI).  Some typical tube replacements for solid state rectifiers are GZ34, 5U4, and 5Y3.  The rectifier controls the power (wattage) your amp can produce. Rectifier tubes, thanks to their "sag" characteristic, produce a compressed character to your sound whereas solid state rectifiers, typically 4 4007 diodes, yield a more tight sound with a lot of headroom. The tube replacements shown, i.e. GZ34, 5U4, and 5Y3, provide progressively less wattage and more compression but you have to be carefull with their filament current requirements: your PT could not handle the job !! Additionally, your power tubes might require a bias adjustement.
Read more on rectifier tube characteristics.

Tip. When looking for tubes, the best ones to get are New Old Stock, or NOS tubes.  These are tubes that are old stock, made when tubes was it! But they're new in that they've never been used before.  You can order NOS tubes from many sources. www.angela.com is a good place to shop.