Do I need a subwoofer for my audio physic sparks?

Hi all, I recently got a rogue audio tempest magnum to go with my audio physic sparks. The setup has a great sound, but starts to break up a bit if i turn the volume way up. The integrated amp only has 60 watts. Do I need something else with more power? A separate preamp? Get a different integrated with more power? Or is the problem with the speakers? Do i need a subwoofer? (I didnt have this problem on my old cheapo yamaha solid state when i turned the volume way up. It didnt sound as nice, obviously, but it also didnt break up at these volumes.)

Any advice would be much appreciated. (I did a similar posting in the amps/preamps group.)