How big's the room ? What's the speakers' efficiency. 60w should be enough for small/medium rooms. A quality amp shouldn't be breaking up unless you're playing very bass heavy music very loud. Something may be broken.
A subwoofer won't help in your situation because there's no way to prevent the amp from trying to drive the low frequencies into the speakers (you'd need separate pre-power for this).
A subwoofer won't help in your situation because there's no way to prevent the amp from trying to drive the low frequencies into the speakers (you'd need separate pre-power for this).