HIGHS - Not overly sharp, but smooth and pleasant. Surprisingly, they reveal tape hiss and noise. The excessive bass makes these headphones seem treble-shy, but it's all there in the right amount. I'd say they sound close to real life's treble. Just EQ some bass out if you want sparklier highs. As is, harsh tracks make these sound sharp enough to bother the ears.
MIDS - As a whole, the mids are full-sounding and well-composed, if a bit colored in the sense of sounding thicker than in real life. Even on low volume, they sound big and enveloping like coming from speakers, not headphones. I am impressed.
LOWS - Drivers this small should not be able to sound so bassy. Seriously, how is this much bass coming out? It's not flabby, but it's a bit muddy, bleeding in to the mids and masking the highs a lot. Good thing is that it has reasonable tightness and speed, and thumps impressively. Seriously, these Porta Pros remind me of how an 8" SQ-oriented subwoofer in a ported small box in the trunk of a car with the rear seat backs up thumps my ear drums. Not boomy or rattley, but authoritatively weighty and present enough to feel the lower octave without sticking out like a coffee-burned tongue. It's not super detailed bass, but it's textured enough to hear the difference in the bass instruments. The lowest sub bass is almost non-existent, though, but I don't miss it, because the rest of the bass is plenty ear-filling.
TONALITY - They sound like good paper cone speakers, conveying sounds with good tonal accuracy compared to real life. Nothing weird about them. I wish my beyerdynamic headphones (DT 880/DT 990/T90) had this realistic tonality instead of the metallic-tinged character they all have. In my opinion, Koss could've easily screwed-up the tonality of these headphones, or chosen to not make them sound so good. But they didn't. Without getting dramatic or teary-eyed, I don't think a sound presentation this subjectively pleasant can be attained by accident or without trying.
SOUNDSTAGE - Better than most affordable headphones I've heard. These drivers are special for sure. Hard-panned sounds sound hard-panned like they ought to sound, but everything else not in the center seems to come from outside the ears and hovering inches over the head. Center channel sounds in front of eyes/forehead, not inside head. The front/rear depth is shallow, but the height and width is smile-inducingly good. My brain is boggled that these seemingly innocent headphones sound so big, even at very low volume.
SENSITIVITY - I've weed-whacked while wearing them with foam earplugs and they got loud enough on my LG G3 on max volume that I could hear the music over the engine noise. One thing I noticed is that they don't need to be enjoyed on a deafening level like lots of higher-end headphones. A bit of volume is enough to bring these to enjoyable levels. They sound good without having to make much noise, like for example, my Denon AH-D340, which on low volume have basically no bass, and on the sweet spot, the bass is perfect, but they have too much upper mids/lower highs for me to endure.
COMFORT - I don't like how they feel on my head/over my ears, but they do not hurt me. The metal touches my big ears sometimes, but I re-adjust them and I'm good.
PRICE - A bargain for sure. Great job by Koss. Long decades later, their formula still shines.
CONCLUSION - They seem so unassuming, but the sound is impressive. I have headphones over ten times more expensive than these, and I don't think of my Porta Pros as my "cheap headphones" but as "my Porta Pros". They're somewhat of a benchmark for me. They're the ruler which I use to measure pricier headphones. I would like less bass on them so the highs would sound more upfront, but other than that, I think they're iconic and punch above their weight. Their marketing says, "Whether you’re listening at home or on the go, the Koss Porta Pro delivers all the excitement of a live performance." As hyperbolic as it might read, I can't disagree with that statement.
Thanks for reading and God bless you!