Dual sub's are something near and dear to my heart. Pick a sub that you subjectively like "just because", and then a sub that technically comes across as a top tier option given your price range. Audition them both and see what your ears think.
My upgrade
I started this quest several years ago. I started with acquiring a Linn Majik DSM, Martin Logan ESL 11A's, I used the Linn as a streamer, Dan and amp and was fairly happy. I also use Kimber Kable speaker wires. I eventually added a Peachtree Nova 300 amp. I used the Linn as a steamer and line source and was a little happier. I added a Parasound JC5 today and am blown away with the sound I am getting. The Linn is now my streamer, dac, and preamp. I will be keeping this amp but am wondering where do I go from here. I'm playing with the idea of a pair of subs either Martin Logan Balanceforce 210's or a pair of REL's advice please