How do I bypass the ac rectifier in a guitar amp so I can power it directly with DC power source instead of mains?
The diode bridge is used together with a center tapped secondary to create plus and minus supplies.  You can't just replace that with a single-output supply.  What you show won't work.

You can get your own &plusmn;9 V supply, then connect its three leads to the three nets you show going into the dashed block at right.  Since the diodes would be reverse-biased if you connect the external supply correctly, there is no need to remove them.  In fact, they give you some protection against connecting the external supply in reverse.