For more details on the TS-590 go to http://www.kenwoodusa.com/Communications/Amateur_Radio/HF_Base_Mobile/TS-590S
Click on downloads and then the brochure.
There are several advantages to the downconversion architecture used. One big advange is that a 500 Hz or 2700 Hz bandwidth roofing filter is used rather than a 6 kHz bandwidth filter. This becomes important during contesting. And yes, on 12 and 10 meters the downconversion advantage is lost. But being that there is no contesting on 12 meters it doesn't seem to be an issue there.
Sorry to hear that about the roofing filter advantage being lost above 15 meters. That pretty much nixes me buying one. I'll just stick with my Kenwood TS-850s.