If you have a good tube amp, then it is essential that you compliment it with a great sounding speaker cab. Not only will it allow the amplifier to reach its full potential tone-wise, but it will allow you the freedom to play your music the way it should be played.
Speaker wattage is an essential consideration when creating an amp rig, and using the wrong kind can have very serious negative consequences on your overall sound. If, for example, your amplifier has a good deal of headroom, and you are a fan of jazz or any other genre which traditionally requires a sweet, clean guitar tone then you need speakers which are capable of handling higher volume cleans, without breaking the sound up and causing distortion.
Speakers are often designed with an ultimate purpose and style in mind, and before committing to buying one, it is important to know if your speaker of choice will compliment your tone, or utterly destroy it.
Rock musicians who often rely on naturally occurring amplifier overdrive in a tube amp will tend to find that lower wattage speakers are better for overall tone and sound quality, as the speakers will respond very well to overdrive and distortion. Try putting a heavy tone through high wattage speakers, though, and all of that hard work setting up your amp and effects for high gain is almost all for nothing.
Another thing which should be considered is the kind of sound coverage you want to have. If you like your bass frequencies loud and clear, then a bigger sized speaker is often better. As the bass frequencies tend to come mostly from the reverse side of the speaker itself, the inclusion of a reflex port in the speaker housing is almost essential for true, rich bass tone.
Check out this awesome article here that provides tips on how to mic up your cab for recordings.