Subwoofers can have the normal unbalanced low-level RCA phono inputs and balanced high-level XLR neutrik inputs as well but the balanced (earthed) sockets are usually reserved for prosumer, high-end, PA and studio sub-bass hardware.
You can also get round the problem of no pre-sub out terminals on an integrated amp with hardwired in-line male-female phono plug/sockets with built-in resistors and/or trimmer pots.
I have also used simple mini passive preamps with high level inputs with good results in the past.
There are 3 distinct types of units but the most common are line output converters.
The uni/bidirectional hardwired phonos with built-in resistive attenuation are not that expensive either - about £20-£50 upwards.
Something like this would suffice
These audiophile devices simply plug into any input sockets and reduce signal levels by 3dB, 6dB, 10dB, 12dB or 20dB.
www.decibelhifi.com.au
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Or one could utilise a passive pre-amp like this
Of course this solution is for an active sub.
If the OP only has a passive sub personally I would put the combined left and right output from the second set of speaker terminals into the LOC in parallel.
One can also utilise a subwoofer bass management preamp with mirrored and high-level inputs and DSP like an Antimode.I had the 8033 years ago and it was brilliant - it irons out the boomy peaks and boosts the troughs - all automatically.It is a lot simpler than the Behringer DCX2496 which is very versatile but requires a steep learning curve, patience and plenty of trial and error.I don`t need a sub management DSP unit these days.
www.dspeaker.com
I have one of these that one can use in conjunction with RCA to XLR leads.
www.behringer.com
Another solution is an analogue or digital parametric equaliser with high level in or a DSP subwoofer bass preamp from a semi-pro/pro manufacturer.