From what I've read the Billie headphone amp is not very good. The amp also has hiss. I have not auditioned one though. The Musical Fidelity amps are dead quiet, sound very good and the build quality is excellent. I have an M6si and it's great for the price but as you know it does not have a headphone amp.
Integrated Amps
I'm debating whether to upgrade my Marantz integrated with a better performing and higher powered integrated or simply add a separate amp and use the Marantz as a pre-amp. I've looked at stepping up to the Marantz model 30 or 40n, PS Audio Stellar Strata, Music Fidelity M5si/M6si. But then the Heaven 11 Billie caught my eye.
Has anyone had any experience with the Heaven 11 Billie integrated amp? It's a hybrid class D amp w/ a tubed pre-amp. The reviews I've watched seem to give the unit high marks for sound quality, presentation, sound stage, and because of the tubed pre-amp, an analog warmth to the otherwise "cold" sound of a class D amp.
https://heaven11audio.com/billie2-amp-features-specs/?v=7516fd43adaa