No need to do either, get a speaker tap cable made and run your headphones directly from the speaker terminals. You'll never, ever need another dedicated headphone amplifier again. I did this probably close to 6 years ago with my NAD M3. The M3 had no headphone input and I just knew that it was such a nice piece of gear that I really wanted to use for headphones. I had Trevor at Norne Audio make my speaker tap cable and I have never looked back. I drove an HD600/650 and HD800S from it, not to mention many other headphones.
I had a 12 foot cable made into two sections and later I had a few short extensions made by PETEREK (not sure if he is still a member here) so that no matter how the headphone cups were terminated (SMC,1/4",2.5/3.5mm) I could use them with the speaker tap cable. Brilliant solution. It isn't surprising that your Marantz receiver doesn't have the best sound from the headphone out, it would most likely just have a resistor network inline which can impact dynamics. You said it yourself that the speaker out put sounds great so why not use the speaker terminals?
The M3 puts out a full 180 watts and I have used headphones as low as 16ohm with it. Well worth considering. Perhaps the most important aspect of amplification is the power supply and a speaker amp such as you have should have a nice enough power supply. There are dedicated threads here about speaker tap cables, maybe do some reading and research the idea. I know that I haven't needed or wanted to buy another dedicated headphone amp since. You end up with a one box solution for both speakers and headphones.