The early attempts at getting adjustable output were amps equipped with a Master Volume control. A lot of them sucked out all of the tone at lower volumes. Later designs use variations on the power-scaling concept, which reduces the voltages applied to the power tubes. This reduces the power they put out, and has the added benefit of making the tubes last longer. Vic swears that his Power Dampening uses a different technique, but really, who cares? If it sounds good, it is good.

For speakers, I recommend getting an >Avatar< cab. Since you want speakers that sound good at any level, I suggest >Weber< California with ceramic magnets and the paper voice-coil dome. Most guitar speakers have a 'sweet spot' combination of EQ and volume that makes them sound good. With adjustable output, you want to use realtively uncoloured speakers with a flat frequency response. Altec 417's are also good for this, if you can find them.

Off the top of my head, I can't think of any combo amps that have sealed-back cabs. Other than my own rebuilt Ampeg >VT-40< that I salvaged from a flood-damaged junker. Notice the Weber ceramic California 15" speaker.