The first upgrade I would recommend is having more subwoofers. Simply to have a higher dynamic range, lower distortion and more amplifiers. That would be the next upgrade step from 1 subwoofer. So, it’s not a transducer, but more subwoofers in the room. They would interact better in the room, the sound field. A transducer or kicker can be great, but I’ve experienced some bad ones, where they act more as a mobile phone under your seat. It doesn’t really feel well, it’s not really well integrated. But I’ve also experience home cinema where the bass from the front speakers was so great, so intense and so deep. That was fantastic! I only learned afterwards that there was a transducer under the seat. It’s all about tuning it, that it integrates well. Tuning means both in the frequency as in the timing. You need to take care of this to have that sensation of better bass instead of something rumbling under your seat.