how to increase power?

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.

thank you for all the respond. i have 2 pair of speakers at 85db and 89db, will a 40watt amp drive them?

i have 2 pair of speakers at 85db and 89db, will a 40watt amp drive them?

Most anything will drive most things, the question is can you get the dynamic range you require.

Theoretically, the 40W amp will give an extra ~16 extra db before clipping. In real life, a bit less. Theoretically, you can drive the speakers at low volumes. Theoretically the dynamic range of the system with 1st spkr pair maxxes at ~100db and the second pair at ~105db spl.

In practise, the short answer is, you're probably better off with a higher power amp OR more sensitive speakers!

There are many other parametres involved that limit the ideal situation -- one of these being how well is the amp dealing with the speakers return, another is the current capability of the amp's PS (because the speaker is NOT a constant 8ohm load at all frequencies).... etc