We are thinking of what to do next, and would appreciate your inputs.  Among the options are

1) A DAC/ADC combination data acquisition system.  This would be 4x 16 bit DACs + 4x single ended ADCs. Ad5764+AD7734 (in progress).

2) Dual channel synchronized DDS.  Two synthesizers, with tunable amplitude, frequency, and relative phase, based around the Ad9854.

3) Higher bit-count precision DACs.  Probably 4 channels, 18- or 20- bit, ultra-low noise DACs based around the AD5780 or AD5790

Please leave your preference, or suggest something else, in the comments!