I have a set of cabinets on the wall in my office, one of the cabinets is completely filled with audio gear now... And I'm working on a second one (half built pMillet NuTube Hybrid Amp, some various OpAmps for Rolling, and other misc parts).
I bought a bunch of headphones too trying to find what I liked.
Basically I learned from discovery and my own testing.
I just kept buying different gear to see if there were differences or if things were "so much better" as many reviews suggested.
I bought a few RCA cables from $0.30c to $30, bought a couple SPDIF cables for optical and for coax, and a SMSL xUSB to "galvanically isolate" my DAC just to test it.
Found that really most everything doesn't make a difference, but some better RCA cables (like those from GhentAudio) have better more solid connections(incase you move your gear), and you can get them in semi-custom sizes that fit and look nice with your DAC (I bought both the standard and 4wire versions)... otherwise I couldn't hear any real difference vs the standard 1.5ft monoprice cables, but vs 50c cheap cables it eliminates hum if the cheap cables are sitting on top of the power cable for the DX7.
So I have a box of various size cables and types of cables, and another box of connectors and adapters. RCA->3.5mm, 3.5MM -> Mono RCA, RCA->XLR, XLR Single -> RCA Single, 3.5mm->6.35mm, 6.35mm->3.5mm, 6.35mm->XLR..
My daily stack is DX7->A30->O2, this is just so that I can have 2 headphones connected without having to swap the DX7 out of DAC-only mode and the O2 is for low impedance headphones that also have impedance response swings which are actually audible.
Daily Headphones are mrspeakers Aeon Closed, or Oppo PM-2 w/ PM-1 Pads.
Also have Denon AH-MM400, Panasonic RP-HD10, 1More Quad & Triple & Dual drivers, Yamaha YH-2, had the Denon D7200.
I've bought too much stuff imho, and really reselling it is hard. I mailed a bunch of stuff to Amir @ ASR to test as he has time, since I don't really use it much (only for audio meet ups).