Options for passive speakers
This won't work out: the speakers in your devices are as loud as they can be with the built-in amplifiers. 

Sure, you could argue that maybe these built-in speakers are very inefficient, i.e. bad at converting amplifier output power to loudness, and you can buy better, higher-efficiency speakers that can make more and better sound out of the same power, but that's extremely unlikely, since mobile devices typically pick astonishingly high-efficiency speakers.

So, with the power you'll get out of these, you're very unlikely to get better sound out of external speakers, unless you're willing to accept less loud speakers, and you won't get much louder speakers, unless you're willing to accept more horrible sounding.

So, you'll need an external amplifier. I don't understand your requirements – you're mostly using them at home (where you have power), but don't want to plug them in – you're making your life unnecessarily hard.