The feedback is likely from the acoustic bass. When they are amplified the vibrations from your speaker often excites either the stage you are playing on, or it positioned behind the bass, the bass itself. Those vibrations are picked up by the bass and cycles through its piezo pickup (or mic, if that's how you are getting the signal) and voila, feedback. Your DNS-112 can handle a lot of power (900 watts peak at 8 ohms), so I don't think it would have been damaged by the feedback.
This has happened a couple of times with my Steinheizer NXT5 electric upright. Never showed up while I was playing, but if I let the bass stand there, it would start to feedback on the open A string. I use my Eden amps for more than one instrument, so now have my electric basses connected to the amp input and my upright connected into the Effects Return, through my Eden WTDI preamp pedal, which allows me to turn the signal off with the footswitch. No feedback since then.
I think you need to find a way to dampen the possibility of feedback by isolating the bass from the speaker cab, probably best done by changing the cab/bass locations.
Have you tried the cab with a different amp? If the hiss is still there, it may be from the cab itself. If not, then it is something coming from your Aguilar amp. I'm not very familiar with your amp and don't know what I/Os it has, but I find that cleaning all the I/Os, front and back, with a cleaner like Caig's DeOxit will often clear up noise problems in amps. I do this cleaning once a year as part of my annual gear maintenance process. Spray the cleaner into each and every plug, then work a 1/4" jack in and out and around in the plug 15 or 20 times. Clean them all, even if not used because often the signal path goes through them even when not used. Crud and corrosion can build up on those contacts, degrading the system.
My Eden amps have a semi parametric EQs, that lets me isolate fairly narrow frequency bands. This allows me to select a frequency band and cut the output there a bit. Didn't work well with my problem because in order to bypass the vibrations coming through the floor of the stage, I would have to cut the low mids just about entirely. Not good for the sound.
I use that feature primarily to deal with bad room acoustics.
Good luck.
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Steinberger NXT5 EUB, MTD 535 Fretless, Tobias Killer B6, 72 Fender Precision
Eden WT500, WTX1000N & WP100, QSC PL230
Eden D410XLT, D410XST, 215, Fender Bassman 10
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