That is a terrible ground... you want the grounding lug to be in direct contact with the bed pan of the car, you don't want it to have to go though the high resistance a washer is going to offer...
You might want to pull your seat out, and peel the carpet back till you can get at a proper grounding point.
You can see my ground wire from this in-progress pic:
It goes up and bolts to the tranny tunnel, that was the best closest grounding position I could find anywhere near the seats.
As far as gain is concerned, according to the V Power manual the "MIN" setting on your gain knob is for 4V preamp inputs...that is what you should technically be using to avoid clipping.
20 Input Gain Adjustment Control
Set the MRP-F600/MRP-F300 input gain knobs to the minimum (4V) position. Using a loud cassette or preferably a CD as a source, turn up the head unit volume until it distorts. Then, reduce the volume 1 step. You can then increase amplifier gain until the sound from the speakers becomes distorted
If you did want to tweak using their setup, I'd start with a pure 0dBFS test tone, I have a bunch available here: http://dave.oc7.org/stuff/test.tones/
I'd grab a 1000Hz (http://dave.oc7.org/stuff/test.tones...BFS%20mono.wav
) tone, put it on a CD and put it on repeat, then with the amp gain all the way down and EQ set the way you want, slowly turn up the volume till the pure tone changes... You'll hear it change from a tone to a slight buzzing sound when it starts to clip. If you can't get it to clip within the headunit (I'd be surprised if you can't), then you can turn the amp gain up a little bit till you find where it clips (keep headunit near max volume and wear ear plugs during this step). Once you know your clipping points, avoid them always =) Turn the amp gain down a degree or two, and never take your HU past the clip point. EG: my Kenwood HU starts clipping at 33/35 on the volume with the way I have it setup, so I never really ever go past 32 unless the album I'm listening to was poorly recorded and has some headroom in it. Most modern electronic/pop/R&B will have 0dB of headroom, so you won't have that problem.
Do the same with a 60Hz tone (http://dave.oc7.org/stuff/test.tones...BFS%20mono.wav
) for the sub amp, though the gain setting should be the same if you are using the same brand/series of amp.