The Marantz slimline avrs just don't have very beefy amp sections, somewhat a penalty for their form factor. They can advertise higher numbers using 1ch driven or 2ch driven so they do (or at specific impedance like 6 ohm, and/or at high THD) and that's allowed somewhat as to how FTC amp rules are written/enforced.
While all channels driven (ACD) ratings aren't particularly important in real use, it can be indicative of larger power supplies used or not....or such can also be limited due to deliberate protection designs as well. It could be a combination of impedance demands as well as power supply in your case, altho the difference between equal ratings of 50 to 70 watts is only 1.5dB. Could also be gain structure and how you set up the avr to an extent as to your experience. Marantz/Denon have avrs where they indicate the ACD will be at least 70% of the 2ch rating. You can also check bench tests of avrs for actual results of power testing.
Your speaker having a nominal impedance of 8 ohms doesn't mean they can't have times where their impedance dips to 4 ohm or have odd phase angles. Try this article
https://www.audioholics.com/loudspeaker-design/understanding-impedance-electrical-phase/page-2
Perfect replacement I don't know....but probably a more powerful amp section than that slimline Marantz, say something from their 6xxx and up series. For Denon I'd check units in the 3xxx series and up, and Yamaha the 10xx and up series.
Another option is to get an avr with pre-outs in addition to that of a sub, so you have external amplification options. The series I mentioned above all have full sets of pre-outs. The Marantz you have does have L/R preouts as well as a sub, so you could take some of the load off the avr that way, too.
