Just curious, where did you read that it's "not safe to hook a sub on the same speaker inputs as the main speakers"? Some context might help, but I'll take a shot at answering for the two most common concerns....
When the same source is used to drive both the amps for the main speakers and an amplified sub (line level input), that is safe. In this case, the main speakers are being driven with the same bass as the sub, so you're not getting the advantage of saving on excursion in the main speakers.
If you're talking about driving a sub input with speaker level outputs, the speaker level inputs of some subs aren't differential, so they connect the negative speaker terminals (of the two channels) together internally. If your amps have "bridged" (differential) outputs, this will potentially damage the amps or sub. Many high performance amps have differential outputs these days. However, there are well designed subs out there with dual-isolated or differential speaker level inputs that handle any amp outputs just fine.
The DAC4800A has non-bridged outputs, so it should work with any sub's speaker level input while it drives the main speakers through the sub. One last "however".... You'll get better significantly performance with ACTIVE crossovers no matter what amplifier you use for the mains.
-Tommy O
