What is a "double DC power supply"?
From the specification:


  
  Supply voltage: double DC power supply +- 58~ +- 70V
  Rated output power: 500W (+- 65V power supply 4ohm load, distortion 10%)
  If the voltage is over 70V, will damage the board
  Recommended double power supply -+65V
  Recommended speaker: 10-18inch 4-8Ohm speaker
  


Working backwards from the rated output:


From \$ P = \frac {V^2}{R} \$ we get \$ V = \sqrt { PR} = \sqrt { 500 \cdot 4} = 44 \; V_{RMS} \$


We need to calculate with RMS to work out the peak to peak voltage which will be given by \$ V_{P-P} = 2 \sqrt 2 V_{RMS} = 2 \sqrt 2 44 = 124 \; V_{P-P}\$

To generate this we would need a supply voltage of +/- 62 V plus a few extra for headroom. Given that the spec indicates 70 V max., you don't have a lot of room to manoeuvre. 

All this assumes that one speaker terminal is connected to the supply common. If, as would be more common, the amplifier drives the speaker in bridge mode then a single-ended supply could be used and polarity reversal done by the H-bridge. In this case neither speaker terminal is grounded.