Companies to look at, with subs in parenthesis
Exodus audio (shiva and tempest)
Acoustic Elegance (AV line)
Soundsplinter (RL-P)
Fi (Q)
Ascendant Audio (havok)
The top three would be my first choices as they all use some form of advances motor/coil topologies to produce lower than average distortion, even for high end woofers. The last two are from the same buildhouse and are simply very good standard designs that will still provide excellent results.
If you want to go used Adire had some nice stuff as well as Ascendant Audio's older line, which was XBL^2 based. Despite that I'd still go with Exodus over the used route as pricing is similar and dan wiggins pretty much designed all of them, but he had more experience to draw upon with his newer lines. Faraday rings are the biggest extra here, along with quieter suspensions.