Subwoofer for Focal Utopia Be's.

Hello, I'm looking for recommendations for a subwoofer to integrate into my current system. Basically, I'd like to augment the extreme low end and add a little punch.

In my room, if I move the speakers closer to the wall, they extend lower, but sound too boomy for my taste. Ideally, I want to keep the Focal monitors away from the wall where they image best and supplement the low end with a subwoofer.

Aside from the extreme low bass, I'm very happy with the overall sound of my system and do not wish to change anything else.

My system is as follows:

iMac optical out
Krell DVD Standard coax out
Benchmark DAC1 balanced out
Krell 400xi integrated
Nordost Red Dawn rev2 speaker
Nordost QuattroFil XLR
Nordost Shiva power cords for all
PS Audio P-500 Regenerator
Target 24" steel stands
2-DIY sound absorber panels at first wall reflection

Thank you,
Gerardo