Personally, I would NOT go Magni with the DT880/600. The DT880/600 is a great headphone from Beyer, probably my favorite, and this includes all the newer Tesla stuff which really aren't all that great compared to the 880/600. The DT880/600 have a nice smooth tonal balance with the exception of being slightly hot in the mid-treble or having a slight metallic quality to the treble. The Magni's tendency to be lean or slightly bright or splashy or steely would not be best. Vali will easily be the winner. Besides, the Beyer 880/600 seem to love tubes. (A great combo with the Vahalla.) The Magni is a good amp for the price. The Vali is exceptional provided there are no issues with noise or impedance mismatches. Replace stock voltage gain op-amp in O2 with LM4562 / LME49720 to take some of the stridency away from the O2.