When choosing a combo tube amp, there are a few factors to consider. Firstly, you should consider the wattage of the amp. This will determine how loud the amp can get and whether it will be suitable for your needs. Secondly, you should consider the speaker configuration. A single speaker will produce a different sound than a multiple speaker configuration. Finally, you should consider the size and weight of the amp. If you plan on using the amp for gigging, you will want to choose an amp that is portable and easy to transport.