Does your standard for flat-itude end at 2k? Listening to too much vintage crap I guess  The treble is certainly not flat so I feel it's misleading to claim it's "extremely flat". Otherwise a great headphone for dirt cheap.

finding a headphone with flat response above 2kHz is extremely difficult, especially at low price points

the only headphones I found with anything close to good upper range linearity on tylls plots were the HD650, Yahmaha HP-1, KRK KNS6400, Beyer DT150, 009, Lambda SB, and maybe HD800 (a bit too much variance on the plot to tell for sure)

all of those are out of his price range or have problems in other areas of the FR so the tascam seemed the best option IMO.

EDIT: Actually, the CAL! is a good option as well: http://www.innerfidelity.com/images/CreativeAurvana.pdf

The KRK 6400 measures reasonably well... but sounds like crap.

Confirmed

OP needs sealed, right? The only one I would add that hasn't been suggested is the beyer DT250. I prefer the DT150, but the 250 is a pretty solid closed, supra-aural headphone.
http://www.changstar.com/index.php/topic,1676.msg44558.html#msg44558

Can usually find it used close to that price range. OK OK, the conversion to fake Canadian dollars pushes it a little higher, but still.
http://www.head-fi.org/t/770395/beyerdynamic-dt250-250   135USD = 175CAD
http://www.canuckaudiomart.com/details/649081204-beyerdynamic_headphone_blowout_all_new_in_box_free_shipping_in_canada/   had them for 169CAD

I sometimes miss that headphone, so much value for money.

Edit: that headphone is a definite keeper when you appreciate its qualities. 

Honestly everything but the FR is pretty awful. I wonder if mods can tame that ringing. Did you experiment with it?

It was a while ago; I don't remember how much I tinkered with it, if at all... pretty sure I just dismantled it for parts. If I remember correctly, not much could be done since it had a capsule/cover behind the driver that basically negated anything you could do in the cup.

I have absolutely no recollection of whether I removed that cover and experimented... which kinda tells me that I probably tried and felt it was so bad it wasn't worth the time.