Just to confirm, medium gain can be maxed out on the volume knob with Oratory1990 EQ preset which has a preamp gain of -7.5 without hurting my ears. Even with the PEACE preset turned off I can still listen with volume knob turned to maximum only with a little discomfort on some sources.
High gain can get to about 1~2 O'clock before getting uncomfortably loud/worrying the headphones will explode.
So, I have to say the Zen Can does seem to deliver a bit more power, but the D10b and L50 stack looks much nicer, and of course measures a good bit better than the Zen stack.
edit - I should add I have Spotify normalising audio at 'normal'. With that off or even on 'loud' even medium gain at full volume becomes a bit painful on the ears lol.
edit2 - After even more playing around I think the biggest change in perception, ignoring raw measurement figures, is the Zen Can having 4 gain settings. Smart move by ifi as it allows more granular control over gain from the medium -> high blow your eardrums out experience of many other AMPs with 3 settings. With high gain measuring a bit worse on this AMP in the OP, the preference is obviously to try and keep at medium gain. I'd say overall with Sundara's this is doable, but ymmv depending on your EQ settings and what volume normalisation you might have going on with music streaming services. Putting the AMP on high and attempting to turn up the dial does reach "ears are bleeding/headphone is about to explode" levels from around 12 o'clock onwards, so any of my musings above are hardly an attempt to say this AMP can't power Sundara's. It clearly can (no pun intended), switching over from the Zen Can just requires acknowledgement of how different the gain settings are.
And finally, for the big question, can I notice a difference between the Zen Stack? Well, the D10b/L50 feel ridiculously transparent leading to there is something different from the Zen Can, which I believe uses a Burr Brown chip. What was more important to me was ditching MQA and getting Windows digital audio volume control back. The L50 also having great single ended means if I pickup non-balanced headphones I don't need to suffer given many other balanced front output AMPs in this price range see single ended as an afterthought.