Simple and powerful audio amplifier around BA4558?
First, if you mention uncommon ICs in a post, provide a link to a datasheet, so that everybody knows exactly what you're talking about.  

You're not supplying a schematic of what you've got so far, but here's one idea.
Since you have two BA4558s you could use them in a BTL (for Bridge Tied Load) configuration. Each IC would handle one channel. In BTL one amplifier connects to one end of the speaker, the other is fed with the inverted signal and connects to the other end of the speaker. Note that each amplifier sees only half of the speaker's impedance.