If this amp used triode output tubes, you could increase power by reworking it to pentode operation. Other than that, you can't expect to make the amp more powerful. If that were possible the manufacturer would have done it.
Why do you want more power? Biamping won't make much difference to loudness: quality might improve. If you want louder you need different speakers.
Why do you want more power? Biamping won't make much difference to loudness: quality might improve. If you want louder you need different speakers.