If it were me I wouldn't worry about it too much. You could try putting a diffuser panel (as opposed to absorption) behind the other speaker to even out the reflections compared to the rack. The GiK suggestion is a great one. If it matters (to you), they even sell real nice DIY kits to save a few $.
Dipole Asymmetry
I am considering purchase of a pair of Martin Logan Summit X speakers. In my room, I am somewhat constrained for speaker placement. I could place the speakers about 3-4 feet off the front wall. My main concern is my audio rack would be placed directly behind the left speaker, while there would be nothing placed behind the right speaker. How detrimental would this asymmetry be on sound quality?