Both are very solid amplifiers, and the SK-4500.1D is run by MANY customers at 0.5Ohms, however we just can't warranty the amp at that ohm load. That is not to say at all that it will not handle daily running at that impedance, again - many customers choose to do it and have had great success. I am pretty sure the Sundown is not warrantied below 1 Ohm either. I think the SK-4500.1 will be nice on your matching Skar ZVX woofers :-)
-Kevin