Keep in mind too that voicing has a lot to do with it. You hear a lot of people say a 100w Marshall will blow away a 100w Mesa Recto. That's probably true, but only due to perception. The Marshall is voiced more in the upper mids which the human ear is more sensitive to. The Mesa with it's lower mid "scooped" focus doesn't seem as loud, even though in reality it's pushing just as much power and probably even working harder because it's driving more bottom end (which requires more power to drive effectively than mids or highs).
Also, wattage is a very misleading term - even more so in terms of tube instrument amplifiers. For example, Orange rates their amps at "clean" power (much like a hi-fi audio amplifier would be). I've heard that the new Tiny Terror actually drives around 28 watts at full song.
And, on the perception issue, guitar amps are generally driving midrange frequencies for the most part. Again, these are the frequencies the humar ear is most sensitive to. That means the 30w you're hearing from a guitar amp seems a lot louder than you might hear from your home stereo driving the same power, because you're home stereo is driving a wider range of frequencies and it's more about peak power than continuous power.
Also, 30w is actually pretty damn loud. When you consider that speaker sensitivity is generally measured at 1 watt 1 meter from the speaker using a constant tone (usually 1khz), and most speakers run in the 95db range (some industrials coming in at well over 100db - JBL's tend to be hyper-efficient for example), and then you take into account that you're pretty much more than doubling the sheer volume with a 30w amp (in SPL terms - roughly 10x power is required to double SPL level, all things being equal), that's pretty freakin loud!