+/- 6dB
eek
that means some frequencies have a 12dB level difference between them. Perhaps H/K chose +/- 6dB rather than +/- 3dB to have an impressive sounding 25Hz extension? Perhaps they have the same bandwidth?
Interestingly, the HKTS-15 I looked up on Amazon.com was rated as "35Hz - 20kHz frequency response (-6dB)".
But back to EddieG, jcPanny has a very good sujgestion... unless you don't want to wait that long for a new 5.1 system.