Yes, sure, but the premise of tablet headphone out and some basic rca cables starts to get more complicated, that's what I mean.Amir just reviewed an active crossover. You can find them cheaper, starting at around $100 USD. Or, you could spend a couple-hundred more dollars and get an Audio Video Receiver which will have a (line-level) subwoofer output and "bass management".
And with an AVR you'll get a bunch more features and you could even use those extra speakers for surround sound. (I like to use a Dolby "soundfield" setting for some delayed rear channel reverb when I'm listening to regular stereo recordings.)
For instance: the xover amir reviewed is balanced, you need to start tinkering with the right cables (not anymore the Amazon basics rca of the title). Then there are single ended ones, sure, you have the minidsp 2x4 very cheap, but people report audible hiss, then you find yourself in the search for something still cheap and hopefully better.... and as you well said, you might end up with an additional box or a full fledged AVR.
I am not saying it is necessarily expensive or too complicated, but once you need more than two channels, it is not always as easy as a headphone out cable to a random Amp and call it a day. I wish it was though