This will happen with many subs with the power of the Z5500. My ambient temp was high because i have a bathroom attached to my bedroom and i left the door open whilest taking an hour and a bit ling shower.
Itll happen anyways but simply take longer without a high ambient temp, i think a good balance of sub volume and main volume is the key. The difference between maximum bars and 3-4 short of maximum isnt too noticeable, i.e. its still hella loud, and i havent had it overheat on the slightly lower setting.
Also, when it overheats it shuts itsself off 🙂 which is nice. I dunno if the effects make any difference to this, when mine overheated it was set to the default 6 channel thing, and not effect PLII Music, dunno if it wouldve made a difference, but i think the effects screw with the voltages and power of each speakers and maybe the sub, like the movie effect boosts the center speaker for example.
Fan idea is very convenient for me actually... it just clicked, my parents bought me a desk fan yesterday.... pretty randomly actually. Good thing they did.