It started with Beyer DT990pros. I bought them, got a Little Dot III headphone amp, which I sold not that long thereafter, and found myself really enjoying the headphone experience as a whole. It lent another, new perspective to essentially all recordings.
Now I own 9 cans:
Beyer DT990pro
Beyer DT880 (600 Ohm)
Sennheiser HD600
AKG K702
AKG K612
AKG K550
AT AD900x
AT A900x
Sony V6 (I've had these for a while. Use them solely for TT setup)
Played with different pads, made some modifications here and there. Some worked out, some didn't. Eventually it all gelled.
I also settled on a few amplifiers, after trying a number:
Violectric V200
Meier-Audio Corda Concerto
And, believe it or not, a 1972 vintage SX-626 Pioneer receiver, whose headphone-out sounds surprisingly good. A shade away from the Corda. The receiver, itself, came from a time warp and that's a story in itself. It as near-mint as a 42 year old piece can get. Came with its original box, packing materials, original bag and polishing cloth, original antenna bound by the original (and deteriorated) rubber band, a schematic, brochures and warranty cards, and of course the manual, but I digress.
I also discovered that an Oppo105's headphone-out is no slouch, either. It's easily on par with the outboard amps I listed, and in some ways more fleshed-out: more air, more space, more low-level detail. With a USB stick, it sounds even better. The 105 is truly an amazing product, IME. In terms of digital, I use the Oppo amp and the V200, which has a forgiving, warm presentation (unlike the 105). The Concerto is for the turntable.
Now my listening is split 40/60 in favor of the main system. I thought I'd put, maybe, an hour or two on the cans per week. Didn't turn out that way. It's the most fun I've had with audio in quite a while.