If you connect the DAP to the input of the amp it goes to a capacitively coupled input to a JFET gate. There is zero possibility of any excessive DC voltage going back from the JFET to DAP to cause damage. Even if you connect the amp output to the DAP output, the capacitors AC couple to prevent any DC voltage from damaging the output stage of your DAP.
Describe symptoms of your DAP and why you think amp may have damaged it - it's highly unlikely except in rare event that you have a bad capacitor. So measure the D.C. Volts at the input connnected to your DAP - should be 0v.
I thought about it, and it may have happened during my 1st build attempt, it was the scenario with bad soldering on the resistors that caused really weird DMM readings, and without knowing better, during troubleshooting, I tried 2 DAPs (Benjie S5 and Xduoo X02) with my first build, and immediately the S5 went into a reboot cycle (and at the time, I didnt know why it did that). After the S5 stuck in a reboot cycle, I checked the X02, it booted up fine and was able to navigate the menus, so I tucked it aside without listening to it, and gave up on troubleshooting my first build.
yesterday, I replaced rail caps on my 2nd build to 2200uF and wanted to see if I can hear any difference in bass performance (and now I know better, after reading through the Objective2 thread that a non-functioning Amp can damage both DAP and headphones, so I pulled out my cheapest DAP, Xduoo X02), and that's when I found out one channel of the X02 was dead, and the working right channel has random static sound.
So during troubleshooting my first build and trying to get some sound through it, that was when it may have happened, probably some AC/DC current gotten into the DAPs (due to short or open circuit somewhere on the amp?) and fried something.