I think that a tuned sealed subwoofer will always give more output than an open air subwoofer, so I wouldn’t expect the Celestion to out DB the Gradients...
what I woul do is spend a hell of a lot of time getting the System 6000 in the right place. As with that much cone and only the ROOM to reinforce the bass if you have them even slightly misplaced the output would die.
I would use the master set set-up procedure, plug in an iPod or laptop, stir off all EQ and start with one sub... then the other, then the monitors..
I’ll link.. it might drive you a bit mad, but it’s Soooo worthwhile, I guarantee you’ll be amazed at the difference. (If not please sell me the subs - they’d go well with my Celestions!!)
https://www.audiocircle.com/index.php?topic=60819.0
Yes, having them push/push is correct and the only way to get a decent output, if they were push/pull you’d get very little output.
edit - hold on. Just read the article that YNWOAN linked to, I assumed from the pics that I had seen previously, that the driver orientation was one facing forward, with the other piggy backing it from behind... this I read is now incorrect and the cones actually face each other? With magnets both on the outsides? Can you confirm?
If so, what you have is a compound isobaric and these drivers must be in physical phase with each other, so push/pull. Make sure that this is correctly wired up, if incorrect you’ll get pretty much zero output.
also when you’re checking that - make sure in turn that each driver Is working - then move onto masterset.