Where would be the best spot to put a logarithmic volume control on this TPA3122 audio amp

I have this headset which I use with a cordless handset.

I'm assuming you will use (or want to use) this headset exclusively with the cordless handset so it isn't a problem modifying it.

If you can get to the speaker terminals then try putting a 100 ohm resistor in series with each speaker. This should attenuate things a bit. If still too loud then try a 330 ohm resistor, Hey they're 3 pence each (at most) so experiment a bit.

If potentiometers are your ultimate aim then experimenting with the resistors will get you an approximate value for a pot ohm wise. Without the little experiment you might be needing a ten ohm pot or a 1k ohm pot.

You ought to consider soldering this modification.

Without doing any real engineering, those in line volume control potentiometers might actually not be such a big deal, provided you insert them in the correct location.

See, the input to most amplifiers is high impedance - meaning, essentially, that it won't draw much current. Using a potentiometer to attenuate the line before the amplifier is a quick and dirty way to get things done, but as far as waste goes, losses around the potentiometer would be fairly minimal.

For several reasons, you certainly do not want to use this hack between a power amp and the speakers. That would be quite wasteful indeed.

When those amps are cranked up, that hum is simply the amplifier doing its job on a signal that probably came from a mildly noisy component or connection. Everything has resistance, and all resistance experiences thermal noise. It could also be power supply noise or a number of other things, but it can only be removed by careful design. If you use the inline volume control, there will now be an additional source of noise that may or may not be tolerable.

For other options, I'm honestly not sure of many other than buying or building a low distortion power amplifier to serve as the input stage to your last power stage. Fortunately, the kind of small one you would need for an intermediate stage can be fairly cheap, since all the heavy lifting is done further down the line.