想請教: 合拼amp 可否改成後級amp呢？

yes, it can

from your photo

left hand bottom side : input,

left hand top side, volume control.

At the middle area, found some op-amp buffer (SIP-9 package may be)

Skip the volume and input buffer. Then feed the signal after the op-amp, that is

Most have 1 capacitor coupling between opamp and power amp stage.

Remove it and feed into is simple