Source : Webster's Revised Unabridged Dictionary (1913)

Subtly \Sub"tly\, adv.
   In a subtle manner; slyly; artfully; cunningly.

         Thou seest how subtly to detain thee I devise.
                                                  --Milton.

   2. Nicely; delicately.

            In the nice bee what sense so subtly true. --Pope.

            Subtly communicating itself to my sensibilities, but
            evading the analysis of my mind.      --Hawthorne.

   3. Deceitfully; delusively. [Obs.] --Shak.

Source : WordNet®

subtly
     adv : in a subtle manner; "late nineteenth-century French opera at
           its most beautiful, subtly romantic with a twilight
           melancholy"