NOTE: I had previously owned, and subsequently sold, the Empyrean prior to being inducted into the Northeast US Tour. The second time around proved a reprieve, not a reprise, for these headphones.

Without reciting my already itemized pros and cons verbatim, I will simply state that at some point in all of my headphone listening sessions, I cannot help but become a critical listener. And that ultimately signals the downfall of the Meze Empyrean in my ears.

. . . luxurious tonality, incredibly large soundstage [but without airiness--go figure] notwithstanding.

My best listening set-up included an after market UPOCC pure silver cable, Chord DAC and hybrid headphone amplifier [3x 6SN7 + 2x Toshiba MOSFETs]: massive--think clear December night sky--soundstage.

Deficient/disproportionately diminutive at the frequency extremes. End of the Empyrean's tenure.

Bass: solid, balanced and extended with excellent pitch. Can be Stygian and prodigious with the right recordings.  May lack mid-bass warmth/emphasis for some listeners.

Midrange: open, clear, coherent, tonally saturated and complex. Vocals are rendered with breath-taking realism.

Highs: clear, detailed and extended with no glare. Excellent sparkle and finish on tambourine, rider cymbal and high brass.

Soundstage: recording- and equipment-dependent. Can assume strikingly large 3-D proportions. The music floats free in space with no sense of closure/constraint.  Superb separation within, and delineation of, the performance space.  
 
Excellent resolution of massed instruments and vocals. These headphones allow you to hear deep--and I mean deep--into the details of the recording, and into the relative strengths and weaknesses of your upstream gear.  
 
I get a very good seal by wearing the headphones slightly low and forward on the ear, slightly swiveled in along the natural angle of protrusion of my ear, and with the leading edge of each ear cup at about my mandible hinge/back end of my cheekbone..

The MLs scale up extremely well, venturing--IMHO--into the "reference" category.  On my home rig, the Mikros 90 even give my HifiMAN HE-6--which run balanced, straight out of my speaker terminal outputs(!)--a serious run for the money!

After initially ordering, I wasn't sure what to expect, but the Mikros 90 have exceeded my expectations with plenty of (head)room to spare. I would rate these, hands down, near the top in the Top 10 list of all the headphones I've ever owned and/or listened to to date. Given their current street price, these are a runaway "no-brainer".

Have not auditioned the KEF M500 yet, but would like to eventually do the a/b alongside the Mikros 90.  
UPDATE: See link to comparative review: http://www.head-fi.org/t/695615/kef-m500-versus-martin-logan-mikros-90

Equipment:
Home:
EVS-modified SONY BDP-S780
Neotech UPOCC interconnects
Fitz-improved Bada PH-12 (2x Tung-Sol 6SN7GT and 1x Sylvania 6SN7GTA)
(or) Linn Intek integrated (headphone out)
Dakiom F-273 feedback stabilizer
Pipeline ET-4 cable
GIEN carbon fiber disc mat
MyAudioCables power cords
PowerVar AC conditioners

Office:
EVS-modified Oppo DV-970HD
Virtue Audio interconnects
KMF Audio headphone amp
Dakiom F-273 feedback stabilizer
Pipeline ET-4 cable
SID disc mat
MyAudioCables power cords
PowerVar AC conditioners
 
Music:
Ultimate Joe Williams
Digital Duke
The Essential Sarah Vaughn
Toots Thielemans: Footsteps
Lee Rittenour: Festival
Cal Tjader: The Roots of Acid Jazz
Dave Brubeck Quartet: Time Out
 
Henri Mancini: Movie Classics
Hiroshima: Go & Third Generation
Herb Alpert and the Tijuana Brass: Classics Volume 20(?)
 
The Yardbirds: Roger the Engineer (Over, Under, Sideways, Down)
Iron Butterfly: Light and Heavy/Best of
Jefferson Airplane: Fulton Street/Best of
Elton John: Greatest Hits & Tumbleweed Connection
Steely Dan: Decade of Steely Dan
Cat Stevens: Tea for the Tillerman
The Marshall Tucker Band: Best of
BTO's Greatest
 
Commander Cody: Best of/Too Much Fun
Earl Thomas Conley: Greatest Hits
 
Baltimore Consort: On the Banks of the Helicon

See a full discussion on the following thread:
 
http://www.head-fi.org/forum/thread/497596/t1-v-dt880-600-round-1.
 
Like The Monkey, I tried gallantly to like these headphones but, for me, they fell short.  While they do lots of things very well, for me there was no synergy among the parts working as a whole.  I ended up preferring the DT880/600 and have since sold the T1 with no regrets.