If you're looking for a low watt tube amp, try
the Egnater Tweaker 15w. I bought one a couple/few weeks ago, & I'm still amazed at how good this little amp sounds. Running it through a 1x12 Greenback cab, it's awesome. Fender-ish cleans, Marshall-ish crunch/drive, & can even do metal with a boost pedal. Best $400 I ever spent on gear. Another plus is you can use pretty much any tube in the power section (It comes stock with 6V6's, but you can use EL34's for example). I'm planning on buying a couple sets of 6P3S's (Russian made), as I've heard these tubes really make this amp shine. The only setting I don't think sounds convinving enough is the AC/Vox tonestack, it's ok, but not as good as the Marshall (Brit) or the Fender (USA).
Of course, it's only 15 watts, but it is pretty loud for such a small amp. I can say if you're looking for the Marshall "roar" (just like Greg mentioned), buy one of the 50 or 100 watt Marshalls & be done with it. While I'd love to have one, it just isn't gonna happen here at home. The Tweaker is plenty loud for what I do, & one of the 50/100w beasts would be overkill for my use, I'd never be able to appreciate/use the power tubes being pushed on a 50/100w amp, so the Tweaker works for me, YMMV.....