The most loud mode of bundle (HD 800s and XD-05 Plus) is Hi Boost On.
xDuuo XD-05 Plus gives The best quality before 108.5 dB SPL (this level between sature color and light color) and it gives maximum volume level before 123.5 dB SPL.
You can choose famous devices and make comparison their loud. This example, HS800s and different AMP sources
http://reference-audio-analyzer.pro/param21.php?&idhp0=706&idamp0=2124&vol0=0&idhp1=706&idamp1=2121&vol1=0&idhp2=706&idamp2=2125&vol2=0&idhp3=706&idamp3=2123&vol3=0&idhp4=706&idamp4=2122&vol4=0&idhp5=706&idamp5=2126&vol5=0&ls=2&tit_graph=2&cl0=%23bb0303&cl2_0=%23bb0303&cl1=%23094a78&cl2_1=%23094a78&cl2=%23029602&cl2_2=%23029602&cl3=%23bb6b03&cl2_3=%23bb6b03&cl4=%238e2424&cl2_4=%238e2424&cl5=%231b415b&cl2_5=%231b415b&style0=0&style1=0&style2=0&style3=0&style4=0&style5=0&mh147=147&ma176=176
The most loud mode of bundle (HD 800s and XD-05 Plus) is Hi Boost On.
xDuuo XD-05 Plus gives The best quality before 108.5 dB SPL (this level between sature color and light color) and it gives maximum volume level before 123.5 dB SPL.
You can choose famous devices and make comparison their loud. This example, HS800s and different AMP sources
http://reference-audio-analyzer.pro/param21.php?&idhp0=706&idamp0=2125&vol0=0&idhp1=706&idamp1=3103&vol1=0&idhp2=706&idamp2=2908&vol2=0&idhp3=706&idamp3=3668&vol3=0&ls=2&cl0=%23029602&cl2_0=%23029602&cl1=%23bb0303&cl2_1=%23bb0303&cl2=%23094a78&cl2_2=%23094a78&cl3=%23bb6b03&cl2_3=%23bb6b03&style0=0&style1=0&style2=0&style3=0&mh147=147&ma286=286
Or different headphones with XD-05 Plus
http://reference-audio-analyzer.pro/param21.php?&idhp0=706&idamp0=2125&vol0=0&idhp1=3808&idamp1=2125&vol1=0&idhp2=719&idamp2=2125&vol2=0&idhp3=1814&idamp3=2125&vol3=0&ls=2&cl0=%23029602&cl2_0=%23029602&cl1=%23bb0303&cl2_1=%23bb0303&cl2=%23094a78&cl2_2=%23094a78&cl3=%23BB6B03&cl2_3=%23BB6B03&style0=0&style1=0&style2=0&style3=0&mh25=25&mh72=72&ma176=176
I think subjective quality HD800s is better than portable XD-05 Plus. HD800s can gives Most quality with different desktop Amps.