There are pros and cons to both tube and solid state amps. Generally, tube amps are preferred for their rich, warm sound that is often lacking in the more accurate, colorless reproduction of solid state amps. The first test of any amplification system is simply to listen to both kinds of amps in order to determine which sound you prefer.

It will also probably help you to understand the differences in how each system works. A guitar, microphone or other pickup device can only produce a tiny electrical signal which must then be mixed with higher electrical power that takes on the same characteristics or "modulation" of the signal to create amplified sound. A tube amplifier (what the english call a valve) uses a heated element inside a glass vacuum tube. The hot element throws off a glow of free electrons that mix with the signal to create modulated power. This system is naturally high in distortion and results in the warm, fuzzy tone of a tube amp. Solid state uses a relatively cold system, based on crystals which have the property of conducting electricity in such a way that the power is modulated by simply passing raw DC power through the crystal at the same time as the unamplified signal. This is why solid state processors are also sometimes called semi-conductors. Solid state, by design, colors the sound much less than tubes. This, of course, is an over simplification, but it gives you an idea.

One point I often make is that the amplifiers currently made with a tube preamplifier are probably the ideal compromise for most users and really do offer the best of both worlds. In most amplifiers, the input signal is too faint to go directly to the output amplifier, so a preamplifier is used. In solid state devices, this is simply part of the integrated circuit design. In tube amplifiers, or the hybrids like Marshall's Valvestate and others, it is a pre-amplifier tube, usually a 12ax7. In the Valvestate and other similar amps, there is both a tube preamp and a solid state preamp, so that you can choose your sound. The important thing to understand is that once the tube pre-amp has colored the sound to tube quality, the introduction of solid state amplification does not and cannot change it back. Since solid state reproduces the sound with virtually no coloration, it simply accepts the sound of the tube pre-amp and amplifies it. Think of it this way: if you are at a major concert and the guitar player is creating a distorted guitar sound, that sound is then put into the concert sound system - which is always solid state. The solid state system simply brings that distorted sound to the entire arena, instead of only within a few feet of the stage. Otherwise, it's virtually the same sound the guitar player is making.

So, do you need a tube amp to get a tube sound? I hate to pop any bubbles, but generally speaking the answer is no. Most people will be hard pressed to hear any difference between a tube pre-amp and an all tube amplifier. Tube amplifiers are more expensive to buy and maintain, and far less reliable, particularly for musicians on the road. Tubes fail like their cousins the incandescent light bulbs, and you must always have a good stock on hand. Even if you check your tubes before every performance, you will still find yourself cutting out in the middle of a show from time time to time. If a 12ax7 pre-amp tube fails mid-song, you simply hit a switch. My advice would be that if you can truly hear the difference or want the prestige and investment value, by all means buy an all tube amp. If all you want is tube sound on demand, a tube preamp will do the trick for less money and with a lot less aggravation.