Slappy, although I've made a comment or two about active and passive subs, I hope you don't think it was me who used the word crappy.
However, I wouldn't hesitate to use the word mediocre. As I don't believe there's one active speaker/subwoofer mfg'er yet who has received any special recognition for their beautiful amplification.
For that reason alone (there are other reasons), I would focus on the absolute best amplifier I could find and couple that with the best pair of full-range speakers that are capable of going down to the low 20Hz's or lower.
As for answering your question, the only passive sub I'm aware of is the German Physics sub which sells for about $13k.
On the other hand, the Von Schweikert VR-4 Gen III SE full-range speakers go down to 16Hz and retail for $6k and some reviewers claim this speaker to be the best speaker under $20k. If you believe them.
If the VR-4 is half of what others claim it to be, and you were able to find the right amp, You'd probably have a bottom end that no sub could approach.