Rel are overpriced, and they look freaking ugly to boot.
Predator doesn't really perform well under 31hz- db output drops off considerably
Music and HT demands are different, for music my sealed box is more than I could ever need- for but HT it's running close to it's limits verging on distortion. I don't have that problem now

went from one sub to three much higher performance/output subs
I would recommend duals, time align them together, then match them up to the speakers, then apply subwoofer EQ. Arendel have app EQ but I'm not sure how you would get the EQ correction to both subs.
I wouldn't recomemend a cheap sub just to say you've got one, a cheap subs (and talking really cheapo subs) will sound bad, poor response, boomy, low quality. I'd rather have no subwoofer than a cheap subwoofer.
Need to match sub to the room. There is no point in getting a one Rel T zero if you have an huge open apartment, and expect it to fill it with bass, high spl and low distortion- without it blowing up.
Mr wolf has a thread on avsforum guide which you work out room volume, expected spl needed and gives out recommendations.