I am using 2 amp for my 3 12" HE2's. The only safe way to run two amps to a single DVC sub is to get 2 of the EXACT same amps. The power rating has to be the same as well as the X-overs settings and the gains. Is the 600W and the 500W your talking about the rms power? The 15" HE2 is only rated for 500W rms (250W / VC) 1000W peak. I would stick with the 600W amp (300W / VC) and go with about a 3-3.5 cube ported box tuned to about 30-32Hz. (Sorry, I can't help on the port size.)
2006 Jeep Liberty
Adire Audio, Cadence, Clarion, MB Quart
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)