You're probably A/B testing different masters.

They are probably deaf from too many loud nights and boost the highs to compensate for their damaged hearing. I've worked with plenty of DJs that do this.

Does it annoy anyone else that spek only has a linear scale for frequency rather than log like every other tool in audio?

If lossless compression didn't work there would be a lot of problems in the IT world. Ever used a zip or rar file?

It's just practice. As a sound engineer I've got fairly good at this because I spend a lot of time with EQ. You start to recognise what common frequencies sound like.

Your post said 'I started ripping my CD's as FLAC at 16-bit with no compression'

None of what you've posted is correct.

FLAC is compressed. Just lossless compression, like a zip or rar file.

Microphone (XLR) cables are designed to be able to run long distances without the signal being lost. Headphone cables are normally under 3 meters so the cable matters a lot less.

Do you have a thread about this anywhere else on the forums? I reckon this is interesting enough to make a new thread about. If we're not careful this thread might end up being all about your software.

Maybe the tracks weren't level matched?

How did you measure the headphones?

That's not how it works.

Audio playback software has one job - to decode digital audio and send it to a DAC. If two pieces of software sound different then it means they are sending different things to the DAC. This means one of them is performing extra processing to the file.

It can hide a load of DSP.   In the real world it shouldn't make a difference, and if it does that's probably a bad thing.