The problem is that as you went higher in the lineup of the reference series, you get the added benefit of more PCM1704K chips AND an increased PSU. The NFB series, on the other hand, only has the PSU benefit without the dac chip upgrade.
Furthermore, for the reference series - both of these upgrades may have assisted in the increased sound quality, but it is difficult to pinpoint which upgrade contributes the most. Even though the NFB7/REF7 has an upgraded PSU compared to the NFB1/REF5, its not like that latter is a slouch by any means.
I'm not presuming the NFB7 is not worth it over the NFB1, but I will not rule out anything until we get more impressions. Likely many others share the same sentiment and feel like the NFB1 is "safe" choice for the time being.