Increase rail voltage by 4 volts(TL494)

I want to increase rail voltage by 4 to 5 volts... I have added picture of the PWM IC.
Current rail voltage is +-28v, capacitors are 35v. So I want to increase voltage to 31v to 33v by changing the duty cycle of TL494
Amplifier is ADS p440

ADSA.jpg
 
The PWM controller for car audio amp power converter could be designed in three ways:
1) without any regulation (why not, it's cheap))
2) with output limit for abnormal power (>15V),
3) with regulation in working range (e.g.11V-15V).
For cases 1) and 2) it runs on maximum duty cycle when input power is within normal range and rails voltage are defined by transformer.
Only the last case allows you to increase rails voltage by changing the voltage sense divider.
You can figure out your case by using lab supply, change input power from 10V to 15V and check if (and how) rails voltage changes.