I have a HDCD CDP, Music Hall, and love the HDCD playback. Besides a one-box, you could go the route of a DAC, such as MSB (under $500). They offer a self-install card, I think around $200. If you were going to stay at the level of a Planet though, and not upgrade, I think cost wise a one-box would make the best sense unless you could get a MF X-DAC w/HDCD for the minimal cost noted above.
Matty