UltraSonic Rx is our default recommendation because of the lower price, though they are not recommended over HAL-O III for any other reason. If cost is not an issue, HAL-O III is a better investment, especially for potentially very hot-running tubes like 211. HAL-O III dampers also work superbly with small-signal tubes even though the extreme heat resistance of the HAL-O III is not needed for them. Audible sonic differences between UltraSonic SS, Rx, and HAL-O III will vary somewhat from one system to another and among different tubes, with neither being inherently superior to the others, falling within a range of system variables and personal preference. Of those who have auditioned and compared HAL-O III with UltraSonic Rx and UltraSonic SS, however, a clear and near-unanimous majority has preferred the HALO-III for best sonic results.
Herbie's Audio lab