For such a low price this headphone deserves 5 stars for sheer bargain prowess! So long as this is plugged into a decent amplifier the sound opens up and is terrific! By contrast, anyone expecting to plug this into their cell phone or laptop computer is missing the point, without decent amplification this headphone will not sound as great as it can(more likely dull, actually!). I am driving this through an IBasso Bushmaster D14, which in turn is amped via an RSA Mustang P51 on high gain. I find this combination with the HE4XX heavenly, also because the P51 almost completely eliminates the Sabre DAC's glare. So my recommendation is to buy the HE-4XX, but only if you drive this through a decent DAC/AMP combo!
i have found using the balanced out of the btr5 far more satisfying in terms of power / drive abilities. that will require an after market 2.5 mm balanced cable however. the single ended 3.5mm out does not drive these planar cans to my satisfaction and there is no headroom (you will need to push the btr5 to a volume of 60 to get a satisfying level). this is not to say it doesn't work with the 3.5mm output, it just doesn't reveal all this headphone has to offer.
4xx for time being right now are $150 and the Sundaras sit at $299 so your saving basically $40 or $50 not including tax or shipping. The 6xx, the 4xx and the Sundara's are all completely different headphones. I only own the 6xx but could say from research I found the 4xx will have more soundstage and bass then the other two. The 4xx will have the less intimate vocals compared two the other two as well. The Sundara will have more bass (you will have crisp bass of a 6xx but actually have some thump or slam that is not present in 6xx's) and soundstage then the 6xx but not close to 4xxs. The Sundara can be with vocals can be intimate but not in your head intimate like the 6xxs. Tonality the 6xxs are mildly warm but the 4xxs are warmer and the Sundaras are consider bright or much more neutral. Good luck it's all in a choice on what are looking for in your next sound be it different or an upgrade based on what you like and feel missing from the 6xxs. The 4xxs will be a more wide ,fun, relaxed and bass with body and slam. The Sundaras will be a more neutral well balanced headphone with some added width plus bass not to far off from 6xxs but with some authority not found in the 6xxs. IMO