What are the recommended settings for the ADAR2001's internal harmonic filters. Can the harmonic levels by reduced further

The performance shown in data sheet figures 16/17 are based on the on-chip filters on the ADAR2001. The filter selection is based on the Table 7 setting vs frequency. Apart from the on-chip filter, additional filters can be added at the output end to achieve better harmonic suppression. However, if you external filter has a fixed pass-band, you will be limited in the frequency range over which you can sweep.