I just got my Sundara and Khadas Tone2 Pro DAC/amp (running it unbalanced which is rated at around 110 mW) yesterday and I'm disappointed with the soundstage. It sounds extremely narrow, certainly more than what most reviews hint at since it sounds almost like the closed back headphones I tried (ATH M40X, AKG K361, WH-H900N) so I am wondering whether they are just underamped or faulty.
Soundstage is something the brain creates depending on the auditory input. Your ears differ from those or others and so does your brain.
Some people hear HP A as having a wide sound stage where others say it is narrow. Could be the other way around for HP B and yet another person.
The Sundara also requires more power than some other headphones and as lows are bit drooping you may need some EQ to make them more enjoyable. This too differs from person to person.
Basically it may be that the Sundara is not optimally suited for you/your ears/your brain. It may be defective but not in 'sound stage'.
That doesn't mean that when others are lyrical (I have seen people going gaga over the HD201) it means it will do that for everyone.
I don't think they are easily 'underamped' unless driven from a phone or USB dongle or USB powered DAC/Amp and don't think it is defective either.
Try it somewhere else or on other HP outputs you could try before deciding it is not for you. Try some EQ as well.
The Sundara is just a decent to good headphone. To some it is the pinacle or best, to others it has no magic and everything inbetween is possible.
That's why headphones are 'personal audio'.