canI mix a sealed with a ported?

The impedance will be different at different frequencies and will send wonky amounts of power to each sub. You could go 2 sealed of the same enslosure size. Or get another amp.

Someone correct me if I'm wrong.

You're not wrong, TC.
But also consider all the phase problems introduced between a sealed and ported sub playing the same frequencies in the same room.

I have ported full range mains and a sealed sub, In phase @ 80hz and out of phase @ 40hz. I needed to set the Xoxer at 80hz( THX setting). Running my mains at full range with the sub doesn't work in this case!!!

You're right, but it doesn't matter, as speakers are driven by voltage swing, not power, and voltage swing is a constant into any impedance. The issue here is phase response, which will be radically different between the two cabs. The OP should just duplicate the existing cab.

Has a better chance of working if the port tuning freq is in the rolloff range of the sealed one

I would do 2 of the same but of course you can mix 2 different designs. You'll need something like a DCX2496 to set them up properly but it can work great if done right. I've done it for years, even now I'm running 6 sea;led subs and 4 subs in an IB. Took some time to get it setup properly but after I did, they will bring the house down and sound great doing it.

Don't shoot the messenger.

The RSS265 drivers are 4 ohm nominal. Parallel wiring a pair will show the OAudio a 2 ohm load, but it's only spec'd for 4 ohm or greater operation. It may not have problems at low volumes, but once you start to get on it a bit, it'll likely self protect (best case). OTOH, the OAudio is rated at 325 watts into 8 ohms, so series wiring would yield ~160 watts per driver.

From the OAudio 500's manual:
"Damage caused by low impedance loads are not covered by the warranty." Where low impedance is defined as any nominal load <4 ohms.

-Brent

I had no issues mixing ported subs with passive radiator, but I had zero luck mixing sealed with either.

Sorry but that amp will not output 500 real watts. I have had 2 of them in the past, good plate amps but nothing special. need to mod them for under 25hz.

My bad. I didn't realize they'd made an 8 ohm version of the 10"...even checked the P-E 10" sub page before posting, but must have skimmed it too quickly.

-Brent

I think you have your amps confused. Airliner's OAudio is good for 450ish per Chas' test, which is a bit more severe than the protocols typically used when an OEM spec's power output. In typical real world use, I'd expect the OAudio to easily meet the spec.

Even the P-E BASH500, which I believe you're thinking of, now comes with a more subwoofer friendly HPF so no modification necessary if you want the f3=18hz/Q=1 also found in the BASH300.

-Brent

Doesn't geddes theory of multiple subs say that multiple you can not only mix different sizes but also ported and sealed?

I am currently running an 8 and a 12 for smoothing with nice results.