If you like your Marantz for the Dolby processing that's fine. The O2 amp being very neutral or transparent should not add or detract from what u put into is within its design.
Most likely the damping factor of the Marantz is helping cause the bass to what it is etch..
You could just try an o2 and odac to see how they compare.
Yeah, I suppose hearing the X1s separate from the Marantz for comparison wouldn't be a bad idea, just to be able to hear how much difference there is in the bass in the first place.
Thanks for the help.