I have the REGA Aethos and had the same issue. Ended up using this volume control.
https://www.ebay.com/itm/264143836049
Connect from RECORED OUT to this then out to subwoofer. Works very well to control the sound level of the sub. Each recording has different bass content so using this small control makes it very easy to adjust bass. Placed the control on top of my shelf within easy reach.