If you can’t hear a difference with the ADI2 set to neutral settings, it should not be surprising. DAC differences are wildly overhyped. But once you turn on the ADI2’s equalizer or use the loudness feature, there will be audible FR changes. IOW, if you like non-neutral settings, you won’t be able to replicate them with the Bimby. I haven’t done a formal level-matched comparison between the ADI2 and my other DACs yet (it just arrived a few days ago), but I have been enjoying the ability to adjust the FR curve, and it makes quite a bit of audible difference.
I recommend pulling up the FR curve for your headphones (try InnerFidelity) and adjusting the ADI2 equalizer so headphone+FR matches the Harman curve (as a starting point). Then do a level-matched comparison with the Bimby. Unless you have headphones which already closely match that curve, there should be a difference. I can try to list some tracks with examples if you need help, but most well-mastered material which strays outside pure mids should work.