Weak preamp voltage for subwoofers when using speaker output

Hi,

I am using a pair of active subwoofers with my line array speakers plug into my Hifi Rose RS 520.

The problem is that the voltage output from preamp is really low, forcing the amps inside the subs to distortion level when trying to match speaker output.

I know that you can increase preamp output in the menu but doing so, that muted the speaker output.

Is it possible to increase the preamp output of the 520 and still use speaker output at the same time?

Thanks for any help.