Grab the M50.. 668 is very good for the price, but can be uncomfortable, especially if you wear glasses..

For me, i would skip superlux 668b, and get superlux 660, it's better and more comfortable than 668b, especially if you wear glasses. For my own experience 668b is very uncomfortable. The 555 is good for jazz, airy and very comfortable. You may want to consider the ath M50

The ath M-50 is a versatile closed phone, Jays Vjays is an amazing ultraportable, superlux hd660, 668b, or 330 are cheap and nice closed phone..

I have borrowed the 660 from a friend last week. Yes it needs amp I never heard DT-770 but compared to my ath M-50 it's brighter, more engaging, staging a bit more congested, quite detailed, tighter bass, but can be uncomfortable if you wear glasses and have large ears..   Oh, headfonia...

I own the M50 for a year and have borrowed the 660 from a friend for a week   The 660 pad is smaller than M50 and can be uncomfortable if you have large ears and wear glasses.. M50 in the other hand is very comfortable, albeit quite warm for your ears.. The 660 is a bit bright for me, M50...

I just upgraded from 448 to ath M50. Quite an upgrade in term of sound quality for me. More bass and treble clarity imho. Also its build quality is just amazing. But the bass can be a little too much for me and can be quite hot to wear it on summer.