- Volume-wise, 50w tube >>> 100w solid state, although I'm not sure why this is the case (probably because of different speakers)
- the human ears are notorious for having the ability to adapt quickly to sound changes even within intervals of seconds, so it's always best to compare equipment, especially amplifiers, side-by-side
About volume, this is what I've found but tube to tube comparison though:
http://www.amptone.com/g112.htmX% louder = 2^log10(P2/P1) * 100%
40 watts is 94% as loud as 50 watts.
30 watts is 86% as loud as 50 watts.
25 watts is 81% as loud as 50 watts.
22 watts is 78% as loud as 50 watts.
20 watts is 76% as loud as 50 watts.
18 watts is 74% as loud as 50 watts.
15 watts is 70% as loud as 50 watts.
12 watts is 65% as loud as 50 watts.
10 watts is 62% as loud as 50 watts.
9 watts is 60% as loud as 50 watts.
8 watts is 56% as loud as 50 watts.
7 watts is 55% as loud as 50 watts.
6 watts is 53% as loud as 50 watts.
5 watts is 50% as loud as 50 watts.
4 watts is 47% as loud as 50 watts.
3 watts is 43% as loud as 50 watts.
2 watts is 38% as loud as 50 watts.
1 watt is 31% as loud as 50 watts.
3/4 watt is 28% as loud as 50 watts.
1/2 watt is 25% as loud as 50 watts.
1/4 watt is 20% as loud as 50 watts.
1/10 watt is 15% as loud as 50 watts.
50mW is 13% as loud as 50 watts
20mW is 10% as loud as 50 watts.
10mW is 8% as loud as 50 watts.
5mW is 6% as loud as 50 watts.
1mW is 4% as loud as 50 watts.
0.5mW is 3% as loud as 50 watts.
0.1mW is 2% as loud as 50 watts.
50uW is 1.6% as loud as 50 watts.
10uW is 1% as loud as 50 watts.