Well if the Revels are like other small speakers the increased need for power and volume, has nothing to do with your speaker, but instead your ears. As you turn the juice up, the speakers bass allignment changes, and provides a few hertz better extension. As such, the speaker starts to sound more full bodied. There are two ways to approach this: Frist, by a big amp a listen at louder volumes. Or second, if you plan on keeping the tube amp then buy a good sub. You can listen at softer levels and still have a full sounding system.
Best,
Best,