ACI Titan Owner: Upgrade?

Hi guys. I've owned an ACI Titan II sub for about six years now. It has served well in both my home theater and my two-channel system. Circumstances may soon require me to buy another subwoofer so that I can have one in each system. ACI has long since gone out of business, although I am aware that a group bought the rights to the designs and I could buy another Titan II or Titan LE if I wanted to.

But while I'm looking at adding a sub so I can have one for each system, what other models should I consider? Ten years ago Hsu and SVS were popular recommendations, but my sense is the HT crowd loves them today while music crowd has moved on to other subs.

As for my requirements, they change, because I move every three years or so. What might be overkill in my current 13x20 room might not be overkill in the next place I live. I owned a Velodyne before the ACI, and found the intergration to be infinitely better using the high pass (speaker-level) inputs than with any other input, but I'm amenable to a sub lacking that input if intergration is flawless without it. Budget is about $2k on the used market, but I'd prefer to stay at $1500 used unless that extra $500 nets me a substantial upgrade in performance. I'm also not adverse to spending $1k twice for two subs to smooth out the performance if that is likely to be superior to a single $2k sub. The new (better) sub will go in the music room, and the ACI will stay in the HT. The speakers in my music system are Ridge Street Audio Sasons. When used with the Sasons I've set the crossover on the ACI in the 65-80Hz range depending on the room.

Thanks in advance for your advice!