It all depends on what SPL levels you desire. Remember that at 20 Hz a signal at 75 db SPL is just at the threshold of audibility...you need copious energy to even hear clean ultra LF sound.
I doubt that the small DD12 will get you much above 100 db SPL below 40 Hz....with room gain you may be up to 105db SPL. To me this is a good match for your Harbeth's.
The Rel's max out at much lower SPL levels - so I would be interested in your findings but I suspect a small model REL would only work in a small appartment in a small room with smaller speakers...I seriously doubt if it could keep up with the Harbeth HL5 below 40 Hz unless the Harbeth's are already doing a lot in this range already and you just need a "top up".
I doubt that the small DD12 will get you much above 100 db SPL below 40 Hz....with room gain you may be up to 105db SPL. To me this is a good match for your Harbeth's.
The Rel's max out at much lower SPL levels - so I would be interested in your findings but I suspect a small model REL would only work in a small appartment in a small room with smaller speakers...I seriously doubt if it could keep up with the Harbeth HL5 below 40 Hz unless the Harbeth's are already doing a lot in this range already and you just need a "top up".