Can anyone hear the difference when you add REL subwoofers when playing two channel

Absolutely an improvement from my experience if done right. 

I don't think you have anything to worry about as far as the Rythmik being "too bassy".  It's only going to sound that way if you have the volume and crossover set too high.  I would consider the F12 as an entry level sub.  You may find down the road you want to add another one (or 2 or 3) or step up to something with more output, depending on the size of your room, how well it integrates, and how much bass you want.  

@m-db

It’s the same as using high level inputs. There is no “magic”. The claim is that unlike a line level connection (RCA), the SpeakOn method allows for the subwoofer’s signal to have the same sound characteristics that is being fed to the speaker. However, it’s pretty silly, what that means is you are thus amplifying a signal that already has been amplified, so it has lots of distortion/harmonics/noise, rather than amplifying a much cleaner signal.

Hi mzkmxcv,

Could you explain your response?  Are you saying the SERVO technology Rythmik is using does not make sense and it will cause distortion?  Jim Salk sure doesn't think so.  There is a reason why he has partnered with Rythmik.  He thinks they work remarkably well and they will provide significantly greater  amounts of bass extension.  In reality, the REL T/9i will be lucky to play down to 35 Hz, but the Rythmik will be able to play down to 14 Hz.  If you are playing a recording that plays lower than 35 Hz, then you will not be able to hear all the frequencies the song was recorded at.  I want to hear everything I can the way a song is recorded.  REL and Rythmik have two different approaches.  However I talked to both Jim Salk and Brian at Rythmik I feel Rythmik will be the right choice.  When I buy something, I want to say WOW and not have to question myself when I can't hear much of an added difference.  Again, if the Rythmik sounds like a boom box, I have 30 days to return them for full refund.  SVS has this same policy, which I feel is very fair.

Let me know what you think after you talk to Brian.  Don't listen to his sales person because he did not design their subwoofers.

You might want to call Brian, owner, of Rythmik.  I think after you talk to him you will realize how much he knows about sound and especially bass extension.  

I really appreciate your hanging in there with me.  I think we will all agree we have all been at a crossroads when looking to achieve better sound quality.  I just wish I had the deep pockets some of you have.  You also must have very understanding wives.  Here is another subject unto it's own.  Why is it there are very few women involved in this hobby.  I know of one in this group and I am so grateful for her to be among us.

Larry, mzkmxcv was talking about the connectors Rel uses, not the Rythmik servo technology.