The first and biggest mistake made with subs, is asking/expecting them to do the work of your primary speakers in the area of mid base.
Most good floor standing speakers, including your R28Fs are quite capable above 40hz - don't expect your subs to do their job.
As frequency levels rise above 100 Hz, they start becoming directional - creating a disconnect between your primary speakers and your sub/subs - if they are not time aligned and in proper relation to your mains. This is why some, falsely, believe that a dynamic sub will not integrate well with a ribbon or stat.
Use the frequency setting on your sub set near the lower range of your speakers - in your case, probably 40hz, but not above the 80hz setting. Set the phase switch, on your sub, at which ever position presents the loudest base (running the sub out of phase with your primaries will cause a canceling effect on the volume and impact of your sub/subs). Spend some time experimenting with the placement and volume levels of sub/subs and room treatments, if necessary.
With a bit of time and work, I think you will be surprised at the difference that a well integrated extra octave of deep base can make in the presentation of your music.
Best of luck...Jim
