The Collaborative International Dictionary of English v.0.48:

Subtly \Sub"tly\, adv.
   In a subtle manner; slyly; artfully; cunningly.
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         Thou seest how subtly to detain thee I devise.
                                                  --Milton.
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   2. Nicely; delicately.
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            In the nice bee what sense so subtly true. --Pope.
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            Subtly communicating itself to my sensibilities, but
            evading the analysis of my mind.      --Hawthorne.
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   3. Deceitfully; delusively. [Obs.] --Shak.
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WordNet (r) 3.0 (2006):

subtly
    adv 1: in a subtle manner; "late nineteenth-century French opera
           at its most beautiful, subtly romantic with a twilight
           melancholy"