Audiophilepure

https://www.dagogo.com/heaven-11-billie-amp-mk3-tube-class-d-hybrid-integrated-amplifier-review/

It is a distinct advantage in reviewing to have two configurable devices side by side. It only took a few minutes to unscrew the footers, lift the lid, swap the board, and close the unit. Kudos for Heaven 11 trusting in the basic competence of the audiophile community and not charging for return (or round trip) shipping and bench time for upgrades. A set of identical components, one with upgrades, offers close to real-time comparisons and chases away any uncertainty whether the upgrades are efficacious; operationally switching between the two units during playback took me about 15 seconds.

As the two Mk3 units were connected identically, the headphone comparison was the quickest I have done. Starting with the music playing through both units, I kept the volume down on them both to silence the mains and subwoofers, plugged the  headset into one of the units, and brought up the volume. To switch, I lowered the volume, plugged the headphones into the other unit with the upgrade Performance Headphone Module ($150 if purchased separately, $99 when ordered pre-configured with the  amp), and brought up its volume.