I would hardly call the Beyerdynamic T1.2 a hidden gem..it's one of the most popular headphones.
Here are a few candidates from me;
Creative Aurvana Live; They have biocellulose drivers, produces a well balanced sound and realistic soundstage. They are like portable Denon AH-D2000's.
Panasonic RP-HTF600-S; Aesthetically less pleasing than the CAL's but arguably sound better. 50mm drivers produces a well balanced, neutral sound but with deep bass.
AKG K612 Pro; No where near as detailed or as perfectly tuned as the AKG K702 (austrain version), but sound very natural and has that classic expansive AKG soundstage.
Symphonized Premium Genuine Wood Headphones; A rebrand of the Meze 88 Classics. One of the best sub $120 headphones I ever heard. Very warm, natural sound like a violin with one of the widest soundstages i've heard from any headphone. I preferred them to Audioquest Nighthawks. The only thing that dissapointed me about the Symphonized headphone was the build quality, I remember the original Meze 88 Classics being of better build quality. I knew about Meze long before they became popular on Head-Fi.
(The original 88 Classics)
Sony MDR 7510/1; Had these headphones for a few days now. I don't know why they never quite took off in popularity, instead remain very much unknown. They are perfect for mixing/mastering music with because of the unique sound signature. Deep, accurate bass with strong mid's and very accurate treble. I find the soundstage to be speaker-like. Most studio/professional headphones at this price don't scale with expensive equipment but the 7510 does. Their sound signature reminds me very much of my Ultrasone Edition M headphones, except the 7510 has deeper bass and wider soundstage, due to having much larger drivers.
And finally, The Fostex T50RP; Not so much a hidden gem as they are quite popular, but these headphones may well be the best choice if you are looking for the absolute best performance for very little money. The drivers are very capable and where they are housed in the headphone, there is much potential to be modded to perfection, meaning it's sound can be improved to the level of other more expensive headphones costing several times more.