Low THD Designs

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I'm interested in designing a low THD, High PF, isolated, 15W driver with a LNK PH.  Using the PIXLs designer tools I can create solutions but the THD is >30% and I can not determe what factors strongly affect THD.  No design solutions tend to yield lower THD significantly.  Are there general approaches to lower THD below 20%, and what parameters have the most effect on THD in these designs?

 An RC network (2.7 Meg in series with 10 nF for Example) connected between input bus and FB pin of the IC provides current into FB pin, which is a modulated signal proportional to the rectified sinusoid input voltage. This makes the IFB current a function of the rectified sinusoid input voltage. As this feed current increases and decreases with input voltage, the input current follows but maintains average output power, thus shaping the input current more sinusoidal in shape which in effect reduces distortion. However, with the RC network connected the set output current computed in the PIXls will change, even if the average IFB is the same with and without the compensation therefore, manual iteration on the values is necessary to get the desired average output current .