Looking around, you have tested LP4, but you haven't tested TP6 LPF2. If this is ok, then you can be 100% sure that the FPGA is working as it should. This is because LP6 is actually on one of the FPGA outputs (page 8-27 on service manual).
If its not ok, then then you know its the programmable low pass filter.
To narrow it down even test TP7 (Atenuator DAC) and TP3(DC offset DAC), and if that is ok, then test TP9, this should be the final output of the source. And if that looks ok, you know its between TP8 and TP9.
The other thing I found was the A5 board is actually part 35670-66505, so maybe that is useful for someone to find the schematic for the board.
The other thing that may help find the schematic is to look at the 35660 service manual (maybe they use the same design in both models).