Generally, people say not to push headphone amplifiers past 3 o’clock. I have been doing this to my Ifi Zen Can and Zen DAC. Personally, when pushed, distortion happens. More experienced people will add more facts to this. I recently purchased the top A90d, and the Q is the same, knowing that the A90d has power control from 1 to 5, and volume control from 0 to 99.
So is it good to push A90d more than 60? In my head, 60 comes at 2 o’clock.
Its fine, the A90 is way cleaner than the IFI zen can (as someone who has both), though i havent found many headphone that need over 65 at high gain ( the only ones are my Hifiman edition XS and my Fostex T60s). Keep in mind that the zen stack strong suit is the warmness of the sound and the bass boost, Topping is much cleaner.
Just make sure you arent damaging your hearing 
So true. Hearing loss is cumulative and permanent, and modern science has zero treatments. My wife has tinnitus from listening to IEMs too loud. It can be maddening and there is no turning off.
That is a pretty powerful and 2:00 is pushing it pretty hard. Do you have any hearing impairment? If not, you likely will soon.
Maybe buy a cheap decibel meter off of Amazon and see what volumes you are listening at. I personally never go above 75 Db’s. You shouldn’t really go above 80 decibels regularly or for long listening sessions.