a Power vs Volume Question

Twice the power is a 3db increase.
10db is twice the volume or loudness.

Both loudnes and volume are perceptual terms correlated with acoustic level. The knob you twirl is a level adjuster.

Not all amps have the same gain. For example, McIntosh, IIRC, rated their MC 60 at 60 watts RMS 20- 20 kHz with a .5 volt input, whereas the Dynaco 60 watt amp required a 1.5 volt imput to reach that level, so its gain was much lower. (I think the days of the MC 60 were before the JASA style sheet required capping the H in hz for Hertz or the B in db for Bell.) Two other things to look for in amp specs is power bandwidth and distortion rating. Those two numbers can make two different 200 watt amps quite different.

As to the OP, for most practical levels and with equal quality amps, there shouldn't be a difference in SPL whether the amp is 200 or 300 watts, unless the speaker represents a very difficult load.

db

each doubling of SOUND PRESSURE level (i.e. spl), i.e. what we "listen to", is 6dB. You need four times the "power" to achieve double sound pressure -- in fact twice "as loud". Yr mic should record +6 dB when your power rises 4times.
In other words... on paper, going from 200W to 300W gives you an extra 1,4dB.

Greg, I believe you are a bit off in your analysis.

Incorrect: Doubling SPL is not 6dB, it is 3 dB,

Correct: Double it again for 4 times and that is 6dB as you stated.

Incorrect: Going from 200 to 300W is not 1.4dB, it is 1.76dB.
dB = 10 log (300/200)
dB = 10 log 1.5
dB = 1.76dB

db growth chart:
85db @ 1 watt
88db @ 2 watts
91db @ 4 watts
94db @ 8 watts
97db @ 16 watts
100db @ 32 watts
103db @ 64 watts
106db @ 128 watts
109db @ 256 watts
112db @ 512 watts

You need to double wattage to gain 3 db
To get a perceived doubling in volume, you need a 10db gain
THX = 110db
THX ultra = 112db
Jet planes, rock concerts and jack hammers usually don't break 130db to 140db or so.
170db could cause a lot more than deafness (brain damage?)
NONE of us own stereos capable of 140db, let alone 170db!

Thanks Herman, I knew there was a log function involved somewhere here...It would be interesting to hear something as loud as 180 db...Isn't the atmospheric pressure something like the equivalent of 164 db? I have heard that the human ear can even become more sensitive if isolated. If you were put into a room with no sound you would quickly be able to hear blood flowing through your body, and air particles brushing your ear-drum. If you were left there for a period of time, you may even be able to tell the changes in atmospheric pressure...

Food for thought,
Ben