I have two BA4558 ICs from an old home stereo. I tested them with simple negative feedback circuit for each channel and with power supply of +/- 6V. The speakers I used are most likely with 8 ohm impedance but that is just a a guess.
The idea is two put together a simple and loud battery powered boombox for a mp3 player. Some hiss or minor distortion is ok. No need for a volume control as mp3 has that.
I guess I could bring the supply voltage up to -/+ 18V as mentioned in the datasheet. A friend of mine suggested adding some capacitors around the opamp and also to add some push-pull transistors after the opamp.
Any other ideas?