I've heard too much. I'll only focus on some of what I've owned, and toss in a few things I remember well from auditions. Otherwise it will take a week to write it all up.
****
Senn HD280 Pro - weak bass, decent mids, narrow soundstage, thin
Senn HD590 - very punchy bass, distant and honky mids, steely highs, good detail, large soundstage and good imaging
Senn HD595 - rolled off on both ends, grainy, good mids, decent detail
Senn HD600 SE - neutral, accurate tone, good detail, airy and open, slow, poor imaging
Senn HD600 Balanced - neutral, accurate tone, good detail, forward, aggressive, impactful, airy, decent imaging
Senn HD650 SE - bloated bass, warm mids, accurate highs, poor dynamic range, good detail, good imaging
Senn HD650 balanced - tight punchy bass, euphonic mids, liquid/flowing character, vivid, dynamic, good imaging/detail
Senn HE90/HEV90 - surprisingly powerful but slightly bloated bass, euphonic velvety mids, scintillating highs, enormous soundstage with amazing soundstage height, very liquid and flowing character, amazing detail, poor imaging with somewhat diffuse images, colored but massively euphonic
AKG K340 stock bass-light - poor bass extension, punchy and tight bass, colored but wonderfully euphonic and transparent mids, forward but linear highs, good imaging, speed, and detail, very vivid but colored sound
AKG K340 stock bass-heavy - somewhat better bass extension, bloated midbass, good but colored mids, recessed highs, good speed and imaging, decent detail, slightly bloated and dull sound
AKG K340 Headphile'd bass-heavy - decent bass extension, well controlled and tight bass, good mids, recessed highs, lacking dynamic range, muffled sound
AKG K501 - poor bass extension and quantity, surprisingly good mids, sharp and metallic highs, good imaging, soundstage, and separation, good dynamic range
AKG K1000 - poor bass extension, very good mids, slightly dry character that can be overcome with system matching, slightly forward but linear highs, massively wide soundstage with decent depth and height, supercharged and adrenalized dynamic range that's very engrossing, good detail and speed for a dynamic
Sony R10 - amazing lifelike mids, slightly loose and boomy bass, slightly steely highs, very good imaging but slightly artificial overall soundstage, remarkable speed and detail for a dynamic
Sony Qualia 010 - amazing speed for a dynamic, amazing detail, very large soundstage with very good imaging, very neutral, slightly but very noticeably wrong midrange tone, very good impact and dynamic range
Stax SR-001 stock - great mids, fastest headphone ever, decent bass extension but very good bass detail, rolled-off highs, forward mids with slight lower-midrange emphasis, great & natural detail, decent imaging, smallish soundstage
Stax SR-001 modded - lots of possible tonal balance variations, open soundstage, better detail, transparency, and imaging compared to stock
Stax SR-404 - good bass extension, slightly loose midbass but good bass detail, lacking lower mids, very emphasized upper mids, slightly elevated lower treble, very extended highs, great speed, great detail, massively open but slightly artificial soundstage, very airy but dry sound with wrong midrange tone
Stax SR-007 - amazing bass extension, great bass detail, weight and heft behind every note, completely neutral, lifelike accurate mids, realistic tone, completely realistic highs, best detail in a headphone but very naturally presented, best imaging in a headphone, good-sized soundstage but completely 3-dimensional with great height and width, incredibly liquid and flowing but detailed sound with great impact and dynamic range, absolute pig to drive and sounds dark, muffled, and with lacking dynamic range when underdriven
Stax SR-007 Mk2 - slightly worse deep bass than Mk1, more bloated midbass, elevated upper mids, slightly recessed upper treble, slightly more diffuse soundstage that's a bit larger but with less precise imaging, slightly less transparent, otherwise identical to Mk1.
Stax SR-Gamma Smeggy-modded & woodied - fantastic punchy bass with decent extension and great weight, accurate and lifelike mids, slightly emphasized lower treble, poor treble extension, great dynamic range and impact, very punchy and vivid sound, great detail
Taket H2 - good bass extension, bloated mid and upper bass, slightly recessed mids, treble can be piercing depending on headphone position, amazing detail, impact, speed, dynamic range, soundstage size, and imaging accuracy, very interesting and potentially amazing but flawed headphone
Ety ER-4P - tight bass with decent extension, decent mids, very steely, forward and metallic highs, poor transients, hyperdetailed, analytical, and bright
Westone UM2 - bloated bass, good mids with lifelike tone, forward mids, recessed highs, good dynamic range
Westone ES2 - great bass, very forward mids with completely lifelike tone, slightly recessed highs, very refined for an IEM, very dynamic, good detail & imaging
Westone 3 - when poorly fitted: bloated mid and upper bass, severely recessed and dull mids, very sibilant highs; when well-fitted: relatively linear bass, very good mids with lifelike tone and transparency, very linear and non-sibilant highs, very fluid and detailed sound
Shure E500/SE530 - very good, deep and tight bass, warm euphonic mids, shouty upper mids/lower treble, lacking refinement, massively wide-open soundstage, good detail, great imaging, recessed highs
Beyerdynamic DT770/80 (pre-2005) - very overblown bass that nevertheless is pretty tight and very punchy, rather recessed mids that still have good tone, emphasized lower treble, recessed upper treble, generally better than they're given credit for being
Audio-Technica ATH-A900: slightly bloated midbass, great bass punch, honky mids with upper midrange coloration, slightly steely highs, very wide soundstage but without much depth and height, good detail, good dynamic range
Koss A250 - great punchy and deep bass, recessed mids with upper mid emphasis, thin and distant mids, steely highs with lower treble emphasis, good speed and detail, large open soundstage with good imaging
Grado SR-60 - poor bass extension, good midrange tone, forward highs, small soundstage, very airy and open, decent detail and imaging, very good value phones
Grado SR-80 - poor bass extension, slightly cold mids, piercing highs, small soundstage, airy, decent detail and imaging, prefer SR-60
Ultrasone Proline 2500 - very deep but boomy bass, plasticky and unrealistic mids, steely highs, good detail, good imaging and separation, vivid but very unrealistic and rather fatiguing sound
****
This is just some of the stuff that I know well, mind you. There has been a lot more. Too much to count.