gains ear

Whats the first sign of distortion from a sub when setting gains? My DMM is playing up, and i want to set my gains. Is it best to use a test tone? What kind of distortion would i hear? Bottoming out? Thanks HU - CDA 117ri Sub - Type r 12" dual 4 ohm -@ 2ohm ---- Ported box 2cbft @32hz Amp -...