upgrading my system and need some advice

currently i have 6 km6000 in boat all wire from the head unit. 4 are wired in series.
i would like to add a skm10 sub box and need some advice on what amp to use.
6 km6000 150w max 50w rms
skm10 300w max 150w rms
the amps i was looking at are:
zx700.5
4ch at 85w rms not sure on max
sub 210w rms not sure of max
or
dx600.5
4ch at 40w rms 75w max
sub 150w rms 300w max

it has been a long time since i have done any stereo stuff but it seems like i would have to have the dx600.5 turn all the way up to drive the sub?????
would the zx700.5 be safe to run if i keep the gains turned way down on everything???
thanks in advance