Setting gain?
 

When setting the gain, should I have the frequency & bass boost set as low as possible and then alter these after, or do I set these before setting the gain?

Cheers

yes you set them at the lowest possible setting and if you can change them on head unit aswell put them both on 0 then put volume on headset at around 3/4 playing a bassy song or loud song then turn the gain up slowly untill it distorts then slightly turn it down. :)

incorrect. play a song that you know well. ie how it should sound. at a high quality recording (ie; from a CD, not an MP3 file)

if the amp is purely for a sub, then it may help to disconnect front speakers to hear the distortion clearly.

alternatively, read the manual with your HU and find the correct output sensitivity, then adjust your gain to that figure (it'll be in volts [V])

i just put down how i was told to set gain in the book i got with my sub