Better is subjective as is pretty much true for any headphone discussion (and lets face it some audiophiles take subjective to faith based religion complete with epic myths and legends);
Still I can give you my utterly subjective opinions on em - the 668B's are studio monitor phones, they've got a very balanced, neutral sound, ideal for EQ'ing and general use but not very exciting - that said it's easy enough to jack the bass in software EQ and get all the clubby bass in your ear you could want.
In fact they are designed to produce basically the same frequency response as the DT990's so if you like that sound, the 688B's get really close. Check:
http://www.superlux.com.tw/upload/fu...8B_view_en.pdf
The 681's are a little bit brighter on the treble, and a bit heavier on the bass, some people find the treble a bit too bright and fatiguing to listen too - there is a filter mod (really easy to do and costs like 50p in components in components) which will passively reduce the offending frequency spike (or you can do it in software/hardware EQ if you like, Drop 7.25 kHz by about 5-6 dB).
The 681's with the filter make a really damn good club and dance music pair of phones - but I prefer the 688B for general use, movie watching and such - So much so that my 5 times the price HD 518's mainly gather dust.
The default fake leather can make your ears sweat, so it is worth buying the AKG k-240 velour earpads, or making your own (which I did).
I can tell the/a difference between the 688B's and the DT990, but it's small (990's are slightly crisper, and slightly punchier) but for a price of £30 ($50) compared to £130, getting as close as they do is a feat.
Oh and there is a new range of superlux which are apparently better all-rounders, but I don't own any (yet).