I think it should be alright if you just watch it. I am running my HC right now with no SSF but only off 200 watts to tie me over till my amp gets back. No issues there. You are just concerned with too much excursion that can dammage the woofer, and most songs don't have that low of bass.
with the 400m youd have alot less chances of ruining the coils in the subs. But with the 600m if you set the gain considerablly higher than necessary you should be fine.
i would go with the 600m. more headroom and you can always turn down the gain. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif