Akuma, no offense but if u have never tried both, you are not in a position to compare them. Also if a 601 has not enough power to drive them, I dunno what will. Maybe a speaker amp?? A 601 can even drive a HD650 and if it can't push a 668B the u better not buy a 668B because the amp u need to drive it will cost more than a thousand bucks.

If u think piercing highs, recessed mids and flat bass is the ideal monitoring signature then ok. 668b is ideal.

For me, a monitor should not have a sound preference and ideally flat. A 440 is nearly there in that it is flat and boring (no musicality or sound preference)

The 668B is way too bright to be a monitor. Simply because sounds from speakers should not come out like that. A monitor should be relatively bright but not to the 668b level.

I do not hate highs. I own a re0 which many praise its highs. But if it's super extremely sibilant then it is a serious issue. It is FAR worse than a dba02 which also has relatively piercing highs.

Stop reading reviews as they are pretty much useless. Try the headphones u wanna buy. Test around to see what you like. If u can't, buy something that has a return policy. Worst case scenario, lose some $$ in a buy sell.