I partnered my ARCAM FMJ A32 and ELAC 205 Jet also with an active subwoofer before my current configuration. After I tried for many times to adjust the input level and cut-off frequency, I finally gave up the bookshelf-sub idea because I could never bridge the gap between them.
Then came my current configuration. I bought a pair of well-tuned floorstanding loudspeakers. The bi-amp significantly improve the bass performance. Although it does not dig as deep as subwoofers, but is more tight and well-controled.
Therefore, I strongly recommend to upgrade, when budge allows, to floorstanding.