- 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.