Pondering REL and Dynaudio subwoofers.

@grantgg I glad you are happy that the REL highly level connection. If it working for you that is great.  I also have a REL sub but with a different issue. I felt that both line and high level signal was too weak for my taste. I am not sure if this was gain issue on the REL or too low of signal from the preamp of the Rotel. Before I had a Hsu STF-2 with plenty of gain.  Yes the REL and HSU are two different subs but with the REL I would increase the gain almost max just to be audible. Could it be a level mismatch  with Rotel preamp, maybe but I found a rare subwoofer processor from Atlantic Technology, SP-8000. This made a HUGE difference not only a gain but how if sounds. This might not be your case but seeing you discourage yourself from a hum issue that you want to go with Dynaudio 3k sub might be better. Go ahead and try it and if it works out in your system and room, more power to you. I had a similar situation with my REL sub. I want to go with a SVS, Rythmik or another higher end Hsu sub until I got the processor. I am totally satisfied with the outcome. 

The Dynaudio looks like a great sub with DSP and presets. Having Dynaudio speakers is a plus with dsp preset for different  models. With any other speaker manufacturer,  tayloring the right parameters might be more time consuming so keep that in mind.  

I have a few older RELs that have a ground lift switch built in and that gets utilized and works perfectly. I also have a Classic 99 that is NOT "slow" in any way, it simply vents downward which gives it a certain tone maybe...one of my subs is an excellent Q108 MK2 that also drives downward. Note the word for low frequencies is BASS not "base."

I don't mean to denigrate the classic 99, it is an excellent down firing sub. I actually started with the classic 98 then upgraded to Classic 99.  If it wasn't for this hum issue, I'd have been a very happy camper.  I just think they they encourage the high-level (speaker) connection over a low leve (line) connection for a reason. In my (probably biased) testing I did feel the high level had a little more clarity.

Quite correct, bass not base.

I have Dynaudio Confidence C2 Platinum speakers and a REL six-pack line array of S/510 subs. I had some hum issues at first, but resolved them. I connect the base pair of the REL sub array to the main loudspeaker terminals directly, but just the red/yellow (hot) leads. I connect the black ground leads back to a common grounding station for my system (a Clearaudio Terra), instead of the ground terminal on the speakers. No hum, no buzz, pure joy.

Grounding issues can be a PITA. Don't blame the gear. Blame the setup and solve it.

With REL subs, you can always connect the high level cable directly to speakers instead of the amp, and they now recommend it as the better option.

@dwette 

 connect the base pair of the REL sub array to the main loudspeaker terminals directly, but just the red/yellow (hot) leads. I connect the black ground leads back to a common grounding station for my system (a Clearaudio Terra), instead of the ground terminal on the speakers.

This is confirmation of what I did on post 6. It seems to work well.