i have a problem with my Head unit and my sub output was just wondering if im doing anythin wrong.

the HU is a Alpine CDE-9828RB
the inbuilt amp is OFF
i have a 4 way sony amp powering my front infinity comps and my rear infintiy 6x9s - this amp utilises the front and rear pre outs.

My sub out put goes to a Xetec Gravity G500 monoblock and the sub is a vibe Space 12.

with my sony old headunit on this setup it was fine however now the sub hardly makes any sound

it is a 600w RMS sub and was VERY loud before no matter what settings i play with i can not improve the sound from the sub

the other speakers are fine

when i plug my rear pre out from my sony amp into my xetec amp the sub kicks into life but i have to fiddle with the crossover settings to get it right.

i want to be able to use the sub out tho as i bought this headunit for the 3 pre outs Alpine are less than helpfull and the manual i have been over and over ?

anyone got any tips ?

cheers