One interesting and somewhat usual move would be to get a couple of 10 inch Dayton Reference HF subwoofers, the 4ohm versions. These have been confirmed to have excellent SQ, and I am sure having two of them will also let them to play loud comfortably. The recommended box size should be around 1cu ft per speaker, but I think they could work comfortably in as little as 0.75cu ft per chamber. You can wire them for the final load of 2ohms. This may seem suboptimal given 1ohm amp capability. However, these speakers have relatively high sensitivity, and I am sure that could make up for the 3dB you lose from going from 1ohm to 2ohm load, and your amplifier will run cooler as a bonus.