Finished my Tiny Giant this weekend, it is indeed plenty loud. (Pics to follow later this week)
At first I thought I had some kind of overdrive going on when going past 9:00 on the (25k) volumepot, but that's only the case when I plug the guitar direct into the amp: going over 9:00 will not output more volume, only more distortion. But with a pedal in front I get waaaay more volume, although I still can't get over 9:00 with the volumepot. Haven't measured the levels yet, as far as I can see the TDA clips at that point?
That could be, it depends largely on what kind of instrument you're using. The amp is designed with some boost in the preamp section, to accommodate a variety of inputs, from weak to hot guitars, piezo-equipped acoustic instruments, etc. So for example my cello with piezo element is much quieter than my active bass - I can turn up the volume half way with the cello with no sign of clipping.
-Use your multimeter in voltage mode to test the voltage at pin 8 of the TL072. It should be approximately 11.6 volts.
That last point is not possible *before* powering up 
You're quite right about that. Thanks for bringing that error to my attention.
Raise the 1k to: 1.2k 1.5k
the output voltage becomes: 13.75v 16.88v
the power becomes: 20w 24w
But wasn't 'stock' (1k) supposed to be 20W already?
The thing is that wattage ratings are - always, by everybody - an estimate or average or a shrug. In precision test applications, an amp must be measured at some incredibly tiny total harmonic distortion to find the "wattage" rating. For guitar, the standard is to measure at some
much higher level of distortion and call that the wattage. So for example, the EHX 22 caliber is claimed to be a 22 watt amp, but I calculated at some point that it's not possible for it to be 22 watts except by a very lenient definition of watts.
Then you have DJ equipment and car stereos which are rated by a standard invented by some truly magical thinkers: this equipment is measured by the absolute biggest peak it can put out, with some monstrous amount of distortion. So you have car amps not much more powerful than the Tiny Giant, being rated at like 900 watts or something else ridiculous.
The amp chip in the Tiny Giant, the TDA7240A, is called by the datasheet, a "20 watt amplifier" - so that's why I've called it that in the literature. In reality, depending on supply voltage and the impedance of the speaker you plug into, it's a 12-24w amplifier. 20 watts is an average since I can't know how you'll hook it up beforehand.
Audio amp ratings aren't standardized, so there's no way to be objective about it, unfortunately. I do hope that the chart at least shows, in an understandable way, how to make it louder by changing the resistor.
Another thing to point out is that double wattage is not twice as loud, only 3db louder, which is on the edge of perceptibility. In other words, don't stress too much about watts. To get twice as much volume as this amp can do, you'd need 200 watts, and no resistor change will get that out of this design, so if something in the range of 15-24 watts is not enough, a 3886 amp would be the next step up.
