That's very astute of you to mention your source, as the input voltage from the DAC does influence the volume heard at the headphones, for any given gain provided by the amp.
So, this is where you need to increase the power output of the PB2 by using HA5002 buffers (which can each provide up to 250 mA of current). What you choose to put in L/R can have an impact on power output, too, as that top row (closest to the battery) is the input voltage gain stage (affecting voltage, not current).
I vigorously recommend that you send a PM to HiFlight (Ron) and ask him to provide information about his "TopKit" for the PB2. He has assembled, and continues to improve, a collection of some very desirable op-amps that are not included among those that iBasso provides. Best of all, he does a great job of soldering the SOIC8 packages to DIP8 adapters, so that you can just plug them into your PB2 - no soldering required. Ask him, but I'm pretty sure he still includes the HA5002 buffer, which should help the PB2 to produce more gain.
While you wait for the TopKit to arrive (assuming you order one from Ron), try powering the PB2 with its 16V wall power adapter vs. the 12.6V internal lithium battery. In any case, no matter what combination of op-amps you roll, 16V power will provide greater dynamics and bass control - especially for inefficient headphones - not at all for the most efficient IEMs.
If you can hear a difference at 16V and are willing to add and carry an external 16V battery pack to your stack for portable 16V operation, just ask for more info. (I'll spare everyone for now.)