I know of 3 amps that have Subsonic/High Pass Filter below 30Hz. The Dayton costs $400, but the RBH costs more.
You can get the Yamaha PX3 Like-New from Amazon Warehouse for $320 (new price is $450).
https://www.amazon.com/gp/offer-listing/B01HIVPOXG/ref=dp_olp_all_mbc?ie=UTF8&condition=all
1. RBH SA-500DSP (500W x 1Ch 4 ohms) - Subsonic Filter from 16Hz to 40Hz in increments of 1dB (16Hz, 17Hz, 18Hz, etc.).
2. Yamaha PX3 (500W x 2Ch 4 ohms ) - Subsonic Filter down to 20Hz.
3. Dayton SA1000 (1000W x 1Ch 4 ohms) - Subsonic Filter Fixed at 18Hz.
