Pro Power is another company. https://forum.audiogon.com/discussions/is-ps-audio-the-only-game-in-town-for-power-regenerators-2 No price provided, but likely as high as PS Audio. They use a battery, so could be a reworked commercial UPS modified to be quite. They do not provide THD output specs, but do have a 17dBa noise spec.
I did find a UK product that was quiet made for audio application, but not sure if they made a 110V version. https://www.powerinspired.com/product-range/specialist-ups-systems/vfi1000qs-silent-ups-system/
With respect to low cost solutions, the APC and Cyber Power UPS units at 55 dBa @ 1 meter is too loud for my room, Liebert UPS are 46dBa again too loud. Would have to have at panel or another room for my audio application. Those unit provide specs of <2% THD. Although per @astolfor measure better .2%. THD with 1.9% THD at panel. I have not measured my THD at panel, but doubt it is >2%. So from my research PS Audio is the only practical game in town.