Been spending time with multiple headphones with the Heretic. Added a HD 400 Pro to my stable.
As for my headphones that pair well with the Heretic, they are the HD 660S2, HD 660S, HD 600, R70x, and maybe the HD 650 depending on the dac. Basically the HD 6xx line in general or something that sounds similar such as the R70x. What doesn't pair well with the Heretic are the HD 400 Pro, T1.3, and the A990Z. The HD 400 Pro is too lean in the lower mids and upper bass on this amp causing an artificial timbre (it sounds better and much more organic on the G111 MKII). The T1.3 is fine in the bass and mids but it's treble gets sharp on the Heretic. The A990Z is interesting it becomes a bloated mess on the Heretic but it's not near as bloated and much better controlled on the warmer and fuller sounding G111 MKII. The A990Z was fine on the Liquid Spark and also the Magni+ (tried it in the past on it) too which is in the price range of the Heretic.
The Heretic is interesting because it sounds really nice on some headphones but other headphones it doesn't sound good at all on. It's more dramatic on how good or bad things can sound on it than my other amps.