Make sure you do a microprocessor reset of the Marantz receiver to clear out whatever settings the previous owner has configured. If you aren't getting a copy of the manual, one can be downloaded for free from the Marantz Web site, and will include instructions for doing that reset.
You can connect the appropriate preamp outputs of your HK receiver to preamp inputs of the Marantz. Set the volume control knob of the Marantz to an appropriate value (0dB if using the relative volume scale) and then run the HK's speaker equalization software to get all of the speaker channels set to the right sound level.
Of course, you might consider doing the opposite, if the Marantz receiver is fully functional and its calibration microphone is included. My understanding is that Audyssey is more sophisticated than the equalization support provided by HK receivers.
In general, however, Klipsch speakers tend to be quite efficient. I doubt you'll hear any difference with the dual-amp configuration. Splitting the load between the two amps would only make a difference at high sound levels.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped