There's a simple answer, and a complicated one. Both are true.
The simple answer is you can add a tube preamp to your combo, no problem. There are literally over a hundred tube pre's to choose from, in both rack and pedal format, at a wide range of prices. Look in the Effects forum for recommendations.
The complicated answer is that not all tube devices sound even remotely similar. Part of that is because of:
--the number and type of tubes used.
--the way the tubes are powered.
--the way the tubes are implemented in the amp circuit.
--the rest of the amp/preamp design
...which can all vary radically from one amp/preamp to the next.
Some tube preamps are designed for high fidelity. Some are designed for heavy distortion. Some are meant to sound fat/warm/vintage. And some are just a scam, with lots of glorious ad copy talking about the amazing fat warm tone, but where the tube doesn't do jack squat.
So rather than focusing on the word "tube", instead try to pin down the actual exact tone you are after (like by naming specific songs, or linking to Youtube clips), and then ask which devices could get you similar tones. You'll get way better results that way.
