Yes, I was never a great fan of the NFB-12 HP out, though I find it has the most weighty bass out of all my amps. The amp section is not bad, just not as good as the other options (Hifiman EF5 and now the Crack). I'm quite happy with the DAC portion though if you get the newer version with adjustable digital filters.
I found the NFB-12 blurs and smooths/blends a lot of the detail on the DT880's (the first cans I tried it with). But detail isn't everything. The DT880's sound a little harsh on some music, so the NFB-12 amp would have been an option to purposely calm the highs at the expense of the detail. But now that I have the 650's, I would continue to use the Crack or EF5 with the 650's on brighter or rougher tracks if I found the DT880's too much.
I'm happy so far with the combo and flexibility of the DT880's and HD650's. I was thinking of the HD600's at first as a complement, but in the end thought the 650's would give more a more marked contrast.
In terms of Crack vs EF5, I haven't spent enough time comparing them to form an opinion on which is better, but I can say that I think both are quite good.