Okay, so here are the measurements I just did with the Spectrum Analyzer first connected directly to the ADF4351 and then filtered, as well as the AD9910. I adjusted the ALC-Circuit to a Level-Control Voltage of 1.25V and 2V Bias-Voltage to avoid driving the amplifier that sits at the output of the filters into saturation.
Please note, the 410 MHz-Measurement is completely off, because the signal path is selected wrong (bug in the selection-routine that I can't fix without an extensive rewrite of the program at this point), but starting at 700 MHz, the correct filter is selected. You can see the effect of the 3 GHz Microstrip-Filter, however.
As you can see, second harmonic is down 38dB, while any higher harmonics are completely gone at 700 MHz - At this frequency the 750 MHz LPF by MiniCircuits is selected.
At 1.2 GHz the second harmonic is down 30 dB - Filtering is done by the 1.5 GHz Microstrip-Filter.
The filters on the new board will have more overlap, so especially at lower frequencies, the suppression of harmonics will be better.
The measurements of the AD9910 show a rather clean signal, except for the constant spurs that are appearing in the spectrum.
Since I don't see those in spectra I've found on the internet by other people, I'm guessing those come from external circuitry on the module I've got here (Cheap module from ebay with 1117 regulators and 40 MHz Oscillator), or the way I've programmed the internal PLL of the chip to generate the 1GHz Sample-Clock - There's quite a lot that can be configured and I have stopped fiddling with those values once I got a signal.
The image signal at Sample-Clock / 2 - Output-Frequency that appears around 350 MHz could be a problem, but the Johanson-Filters claim to have -30 dB attenuation at that frequency and I'm using 3 in series, so that should provide sufficient suppression of those signals.