Novice Question: Adding a subwoofer in parallel?

Hi all! I’m putting together a budget system to act as monitors for mixing music.

I have 2 x Alesis Studio One MK2’s, that each have an impedance of 4ohms, and I have a Pioneer VSX-9500S receiver which can power those monitors at 4ohms.

I am being given a B&W Acoustitune sub for nothing that I would like to try adding to the system, but I wanted to check to make sure that I’m not going to blow my receiver.

The Acoustitune has what I think is called “high-level” connections, so it connects directly to the receiver via the 4 +/- L&R “A” output terminals, and then feeds the signal from that sub to each of the monitors.

I seem to have found conflicting information. The sub states on it that it has an impedance of 9ohms. Would wiring the speakers and sub in this way actually end up lowering the impedance and potentially causing damage to the receiver?

Thanks In advance!
 
Haven’t been able to find a proper manual anywhere which is why I came here. Image I could find here:
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One problem you may run into with a Passive sub is that the relative loudness of the sub may not match the stereo speakers. The sub may be louder or too quiet compared to the stereo speakers. There are no level controls. Thats why most subs now adays are active. I was given a passive sub and I ran into that same problem. Its sitting in the corner collecting dust waiting for a sub amp with controls.
 
Unscrew the back panel. Post a pic of its reverse. We need to see any electronics in between the input & output terminals. If B&W are true to form I suspect you'll find a couple of caps and circuitry acting as a high-pass filter to your Alesis. The overload protection on the outputs indicates this to be likely. If that's the case you'll be okay.
 
After all the comments and input I turned the sun down in the end as the other party changed their mind and wanted cash for it. Only $20, but I felt even at that it wasn’t worth the hassle for something that might just end up collecting dust until I could pawn it off to someone else!

While I’m here though, I have seen another sub come up that might be interesting, with the same kind of issue - being that it uses high-level inputs. It’s a JBL PSW-1000.

Does anyone have any experience with that unit? Is it worth $20 I’m this same kind of setup situation? This has a phase switch, crossover knob and volume control so a bit more control over how to integrate it. Any thoughts? Thanks!