Last time my hi-fi installer set-up my DLS amp and Sub they installed a rotating switch which came with the amp, is this needed as my Alpine head unit also has a control (gain) for the sub, or should both me used together? I'm a little confussed...

 

Yes, it is troublesome. I use the head unit sub gain as a "volume control" for the sub, as it allows me to turn the sub down easily on very bassy tracks or leave it high on quiet bits.

You could theoretically get a better sound by whacking the gain on the h/u to its maximum, and then turn down the gain on the sub amp, but given that your amp will filter high frequency (where noise is most likely) then it'll do nothing.

I'd go with middle setting for gain on the h/u and set appropriately with a dynamic track.