In reality, I'm not necessarily sane about this either.
My affordable threshold for micro-SD cards is $0.50 per GB. By that standard, I've been able to afford a couple of 128 GB cards, plus I have 4 * 64 GB cards available for music.
But all my e-tunes are on an external hard-drive. Which is synched regularly to a backup, which is a 2TB, USB-driven, ruggedized portable drive, about the size of a deck of playing cards, and that is (so far) large enough to hold all my e-music. (Toshiba made this particular HD.) On long trips, the PC and the portable drive go with me, plus an iFi Audio Micro iDSD DAC/amp. If I get bored with what's loaded on the sd-cards for the DAP, I can load different music, mid-trip.
In the spring, I took this rig on a ship cruise in the Caribbean, along with my decent cans, an HD-650 and an HiFiMan 400i. GOOD TIMES !!! I was literally about 10 minutes into joy time on the first day ... we'd barely left port ... when my wife said to me "You know you're not going to be able to listen to that stuff while anyone else is in the room, don't you?!?!" (aaaaaah! :eek: :eek: :angry_face: :mad: )
I bought some closed-back cans (Oppo PM-3) almost immediately upon getting back.
Have you tried the PM3 with the QP1R? Can you describe your findings please. This is a closed can I am interested in. Thanks