Note for this review: I found the sound of the M-80 less energetic in the high end compared to other headphones I have**, so I used the iPod's treble booster setting for most of my tests and comparisons. The overall sound quality of the M-80 is so good that you can play it without EQ if that proves satisfactory, or try different settings to better match your personal preferences.
**Those other headphones I have are considered to be "bright" by some users, and fortunately the added EQ does not make the M-80 sound that bright. Note that the iPod's settings remain active even when connected to a headphone amp through the iPod's docking port.
The one headphone I've used that to me sounds most similar to the M-80 is the Beyerdynamic DT-1350, but the DT-1350 has a much greater midrange emphasis than the M-80, and so I think the M-80 would be much easier to adapt to a wide variety of musical tastes.
The bass of the M-80 is tight and detailed, but 'tight' in this case does not mean light or thin bass - the bass is prominent and full on any track that actually has bass. I'd rate the M-80's bass as 9 out of 10, and since I haven't heard a 10 that means the M-80's bass is as good as I've heard with any other headphone.
The midrange is more difficult to judge since nearly every aspect of music is involved with the midrange in one respect or another. I can say that after playing nearly 100 of my favorite tracks in many different genres, the midrange sounds very good - voices sound right, instruments such as guitars, cellos, trumpets, pipe organs - in short everything I've played - sounds marvelous.
The high end of the M-80 sounded less prominent to me than the midrange and bass as I noted above, but what's interesting here is that even after I added treble with the iPod settings, there is still no problem with sibilants or other harshness. I wanted to make sure that this wasn't the case due to high-frequency rolloff, so I checked my lineup of test tones from about 4 khz up to about 13 khz, and those tones were audible in their proper proportions.
I can't give the M-80 the same kind of rating for midrange and highs as I gave for the bass, since the bass is 100 times easier to rate than the mids and highs. But the M-80 is the best-sounding portable headphone I've used so far, and is second in sound quality only to my large desktop headphones, the Shure SRH-1840 and Philips L1.
The M-80 will play quite loudly with portable devices such as most cellphones, iPods and so on. The two straight cords are detachable, the miniplugs are at a 45 degree angle, and the cords are covered in a fabric that doesn't snag on my clothing, which is a major plus for me. The carrycase is compact enough to include in carry-on luggage on most U.S. flights.
The physical quality of the M-80 is as good as the literature says it is. Comfort is good, with much less pressure on the ears than the Beyer DT-1350 for example. Isolation is moderate though, so don't expect it to block outside sounds to a major extent. Soundstage is average, which is better than I expected for a small headphone with small earcups.
In addition to the pop music tracks listed below, which I used mainly for detecting weaknesses or other problems with the sound, I played a wide variety of genres (Jazz, Diana Krall, Bill Evans Trio; Bach organ, Biggs; Beethoven 9th, Solti CSO; Chopin, Moravec; Reggae, Marley, Tosh; Country, Haggard, Yoakam; Verdi, Domingo; Sinatra and Bennett; Punk, Germs, Fear, Sid Vicious, Social Distortion; Medieval, Madrigali, Medieval Babes; Trance, Mylene Farmer, etc.)
The following are some of the music tracks I tested with, and the main features I listened for with those tracks:
Blues Project - Caress Me Baby (piercing guitar sound, handled very well).
Cocteau Twins - Carolyn's Fingers (guitar string detail and quality, excellent).
Commodores - Night Shift (bass detail, very good).
Germs - Forming (raw garage sound, excellent).
Lick The Tins - Can't Help Falling In Love (tin whistle, very clear and clean).
Lou Reed - Walk On The Wild Side (bass impact, very good; detail very good).
REM - Radio Free Europe (drum impact, excellent).
Rolling Stones - She's So Cold (bass impact, excellent; guitar sound very good).
U2 - With Or Without You (bass boom/high-pitched instruments/sibilants, handled very well).
Van Morrison - Into The Mystic (bass, very good).
Who - Bargain (voice trailing off: "best I ever had", very good vocal harmonics).
Now that I've presented the worst-case scenarios for the M-80, it's time to get into the musical qualities in greater detail. At this point in my listening, the highs are smooth and extended, and the midrange sounds just right. Then there's the bass. Ooooh, the bass. Compared to other portable headphones I've had, the bass is tighter and more detailed, yet has greater impact. I can not only hear 15 hz softly and 20 hz strongly, but I can hear the beats of those frequencies, albeit at 20 hz it's too rapid to hear as a clearly enunciated staccato sound. And this is not just harmonics playing tricks - the 15 and 20 hz fundamentals with low distortion are obvious and well defined.
Comparing the bass of the M-80 to the Shure SRH-1840, the 1840 is comparable, but sounds slightly weaker. That comparison is with each headphone running from the Objective2 headphone amp. The good news in running the M-80 from a decent headphone amp is that the highs open up and there's a better sense of space and clarity, particularly on very dynamic passages. And the deep bass gets tighter, yet does not lose any strength or impact, which continues to surprise me with the M-80.
My overall summation is that the M-80 is an excellent music machine in most respects, except that it's a phenomenal bass machine. I have never experienced bass that goes so deep and remains so powerful, yet is never boomy, bloated or other such negatives, no matter what I play. Note that the M-80 is not a "bassy" headphone, nor does it impress me as being very "warm". It simply goes very deep and retains better detail and impact than any other non-bassy headphone I've heard.