I don't know the specifics on that amp particularly, but I've been looking into the ABM amps lately (which also have a sub out connection) and this feature intrigued me.
I believe (from the manual) that the "sub out" is an output with a low pass filter @ 160 HZ . So, it outputs ALL of the signal below 160 HZ, whether or not you have the subharmonics on or not.
If the subharmonics are off, you still get that part of your bass tone that is below 160 Hz. I don't know if it is pre or post eq - I suspect post though. The idea being you can "biamp" with a second amplifier to add bass power support. So in that case I believe it is a line level signal.
So, for recording, I'm not sure if it gives you much, since it is just like doubling the track and cutting out all the highs.
For live play, with a second amp, however, you would get some pretty serious "bass magnification", since you could have as much power as you want just driving 160 Hz and below.
Again, this is based only on my interpretation of the manual, not from actual experience.