Being in Australia is a whole different ballgame. You don’t have the luxury of “risk-free”. Sadly, I’ve never heard a Rythmik sub. I really wanted to audition them and was willing to pay the price for shipping both ways to do just that, but there was a six-month backorder on the subs I was looking at when I was auditioning.
I think you have two very good subwoofers that you are considering, so I doubt you’ll hear a big difference between them. Both are relatively small sealed designs and both should provide you with solid bass for HT. Rythmik has a reputation of being a very good subwoofer for music, and their FV15HP has always been consider a great performer for a large ported design.
Perhaps you need to find out the trial and return policy in Australia before making your decision. Either way, I wish you the best.
Music area: Magnepan 3.6, McIntosh MC2205 amp & C48 preamp, SVS SB13-Ultra, Oppo BDP 95, dbx 3BX, and assorted equipment.
Movie area: EMP Tek E5Bi (were rebadged to R5Bi), RBH/EMP Tek R55Ti, PSA S3000i, Denon X2000, Oppo BDP 83.