I have Sundara, HE560, SR1 and HD650. Going from brightest to darkest. HE560-Sundara-HD650-SR1
I find Sundara to be superior to HE560 mainly because it is more balanced. It is also more easy to listen because less the treble "shizzle" which gives more space for the other qualities to shine throug. HE560 treble is a bit problematic (unless you have a tube/tube hybrid) but Sundara does not have those problems. Atleast not to that extent. Still I feel Sundara leans more to the brigher side and I would not call them "VERY neutral" as PinkyPowers said. Defination of neutral varies from person to person of course, but to me Sundara has too much treble and lacks some bass to be call perfectly neutral. It is an insanely good headphone, whatever you call the soundsignature.
Here is Sundara measured with HE560. Volume was not perfectly matched, but you get the idea.
Sundara with HD650
Ibasso SR1
I tried to get new measurements of SR1 as the one above is with stock pads and I prefer PT1 pads. Something is wrong with my measuring equipment so it messes up everything above 13k. So I can't do a fresh comparison measurements. Do notice that it is taken in a different scale. It looks like the bass rolls of soon, but it rolls of after 10hz (!) where as other measurements begin from 10hz. Sundara, hd650 and he560 were measured on a same day one after another, so those measurements are valid.
Anyhoo... Of Sundara, HE560, HD650 and Ibasso; I prefer Ibasso SR1. Hifimans tend to lean on the bright side and for me SR1 is more natural sounding. Even though it is darker it is equally clear on the mids and has punchier bass. As measurements show, hifimans don't even have the edge of linear bass down to lowest hertz because SR1 is one of the few dynamics that can reach 10hz with linear response.