UltraSonic Rx and HAL-O III are both state-of-the-art vacuum tube damping instruments. HAL-O III is not necessarily an upgrade over Rx. If the Rx dampers are doing well for you, there's no need to change.
Subtle differences in the performance between tubes using Rx and HAL-O III will vary somewhat from one system to another, between different tubes, and according to personal taste. In most cases, however, results will be equivalent with the tubes simply performing at a higher percentage of their own potential.
As far as general trends between the two, HAL-O III tends to be very revealing of system strengths and weaknesses whereas UltraSonic Rx can be a little more forgiving. HAL-O III will sometimes have a long-term advantage with extremely hot-running tubes, though tubes too hot for Rx are extremely rare.
Because of the price advantage, UltraSonic Rx Damping Instruments remain our default recommendation for all hi-end audio system vacuum tubes.
Herbie's Audio Lab