The first version of the R-DES had an adjustable subsonic filter. Here's the manual describing it:
http://www.av123.com/manuals/31_R-DE...smanual_v1.pdf
The newer version doesn't have it anymore. But I'm guessing there is still some natural VLF roll-off being present. I haven't seen a single electronic device being dead flat down to DC. It would interesting to see it measured. Maybe you could DL the newest version of the REW (I'm sure John can give you the link) which goes down to 2 Hz.