[leoedin]

This is cool! I wonder if you connected just one of the audio outputs to a speaker with a crossover, the system would adjust to put the whole frequency range through it?

It may also be that there's somewhere on the pcb you can tap into line level full frequency audio (or maybe not - they could easily do the filtering only in the digital domain)

One thing to keep in mind when doing this is the impedance of the speakers.

I'd measure the resistance across the IKEA ones and whatever speakers you're using first. It may be that the IKEA ones are not the common 8 ohms. If they're more (eg 16 ohms) there's a risk that an 8 ohm speaker will result in too much current through the amplifier.

[phasetransition]

A speakers' impedance has substantial reactances, and the nominal ohmic ratings have little practical meaning.

Almost all modern class D amplifier ASICS have short circuit protection on their outputs

[pjc50]

The description of the tuning app makes it almost certain that the crossover is purely digital, possibly integrated with the codec as well.

[KozmoNau7]

If they're built like other active speakers (studio monitors and the like), the crossover is line-level and there is a power amp for each driver. So any full-range signal you get is going to be at line level, not amplified.