Im talking spirited basement playing alone on a reasonable cab... say a 210. From what I've seen, the extra wattage really shines once youre at a higher volume or with much larger cabs. My th500 was a great bedroom amp and rocked for smaller rehearsals but there was always something missing from the sound.
I've felt the same, and I think a lot of it has to do with the compression / unclarity / distortion that happens in the room and to your ears as the volume goes up. At low volume, the lower wattage (tube?) amp has some mojo from being pushed a bit, and the larger amp is just coasting, probably at it's lowest distortion. As the volume goes up overall, the larger amp stays clean, and fares better against a loud and muddy room or stage.