Encoding DNxHD using ffmbc
1) If you're not going to deinterlace it then stick to the source's field order.

2) I wouldn't bother with the two pass encoding (if indeed it even does anything) - dnxhd will only encode at certain specific fixed bit rates anyway so you're actually pretty constrained for options.

3) You'll see some softening, but it's unavoidable.  ffmbc's filtering is ok though.

What software are you editing in?

And the 185M bitrate might be overkill if your source is HDV.  Try 120M and save yourself a bit of space.