The double amping is not for volume, but fidelity. A dozen critical listening A/B's using well mastered lossless files of various genres, between the QP1R solo, vs. stacked with my matchbox sized FiiO E06 amp shows that the treble presentation is somewhat subdued / harsh and dynamics mediocre with sondstage that is 'meh', vs. when paired with the FiiO E06 it was the opposite: slamming clear dynamics in spades, wider soundstage and very smooth / resolving highs. And I tried L/M/H gain settings on all headphones, to which high was always best.
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Unfortunately the reason I got this is to avoid double stacking in the first place, but the 'direct' sound is still better than my FiiO X5 v2 at least, so I'll see what to do.
Probably others have found better external amp pairings for this as well. Anyway I'll be away for a while now.
As always YMMV...
