Dom't get hung up on Amp Power Rating. In Subwoofers, we have no knowledge of the Driver Sensitivity rating, thus no way of knowing how efficient the Driver is at doing it's job.
Most important are weighing other metrics, if available. Being able to see measurements of Burst Output and Max Output Over Time along with Compression and Distortion measurements tell the story. Group Delay measurements are also helpful, but most Subs on the market today do pretty well at GD where it matters.
I'm not familiar with the Subs you mentioned. I know there are reviews here on AH for the SVS and possibly the Starke Sound.
XTZ is more European based and we don't see their gear in the US, though in general XTZ does have a good reputation as best I can tell.
If you reach out to XTZ, perhaps they can share some performance metrics with you that you can compare against the SVS and Starke?
Happy hunting!
