... the ability to finally drive the bass enclosures of my 7se's like they were made to be driven means I should no longer need my sub (except maybe for movies).I suspect that if you connect the sub you would be connecting it to your Integra processor, and using the preamp's cinema (home theater bypass) mode. If, however, you ever find yourself wanting to connect the sub via its speaker-level inputs, an important point to keep in mind is that the sub's negative speaker-level input terminals must not be connected to the negative output terminals of any of the amplifiers.
Since the DNA-500 is fully balanced, and the 300.1's are being operated in bridged mono mode, all of their negative output terminals have full-range, full amplitude signals on them, rather than being grounded. Connecting the sub's negative input terminals, which presumably are grounded, to the negative output terminals of the amps risks shorting those output signals to ground (depending on the internal grounding configurations of the sub and the amp), and causing damage.
If you want to connect the sub to the outputs of the DNA-500, either of the negative input terminals of the sub should be connected to a circuit ground point on the amp. Chances are that a chassis screw is such a point. That could be verified by using a multimeter to check for continuity between the screw and the ground sleeve of one of its rca input connectors, or between the screw and pin 1 of an xlr connector (while nothing else is connected to the amp).
If you want to connect the sub to the outputs of the 300.1's, since there are two of them one of the negative input terminals of the sub should be connected to a circuit ground point on the preamp.
The two negative speaker-level input terminals of the sub are most probably connected together in the sub, so only one of them would need to be connected externally. The positive speaker-level input terminals of the sub would be connected to the positive output terminals of the amp.
Regards,
-- Al