Subwoofer

Ask the manufacturer.

From my experience, subs that have auto on without a hard on/off switch are pain because thats what they do: shut of in the middle of a listening session because there is no bass for an extended period: No matter how much you fiddle with settings an auto on sub almost always turns off unless you are listening to hard rock. Most of time you would have to use a high crossover setting that might not blend well with your speakers. The few auto on subs that I have owned have been my least favorite. 

Rel makes fabulous subs.  I have a pair of S/5 SHOs and in addition to adding weight and expanding the sound field, they absolutely disappear.  Subs should crossover in the 30 to 40 hz range.  They are subs, not woofers.  Much higher than that and they won't disappear and will give you a boomy sloppy bottom end.  

If I remember correctly the Sony has a gain adjustment on it. If my memory is correct I would suspect you have the gain set too high for the sub and it just isn't able to handle the signal. Again, if I am correct about the Sony having a gain adjustment on it I believe this is the same pre-pro that a friend of mine kept blowing JBL speaker surrounds out with because the gain was set too high.

Had a look at the manual for the Sunfire True Subwoofer MK IV and there is unfortunately no option to bypass the Auto on/off function. Because newer movies are usually Avengers-type they are full of low end, but if you watch dramas or listen to non-metal music, you are going to have issues. The maximum crossover on that sub is 100hz, so you are already almost maxed out.

My advice is to dump the sub and by a used Rel 5 or Rel 7.