After I recently bought a SunSDR2 PRO I wanted to connect it to my Expert 2K-FA amplifier. The best solution I found was connecting them using the Yaesu-style band-data bits. You need to set the amplifier to Yaesu Band-Data.

Pin layout:

SUBD15 (SunSDR)     DB15 (amplifier)

2                                      14 (Band-Data B)
7                                      6 (Band-Data A)
8                                      15 (Band-Data D)
12                                     7 (Band-Data C)
13                                     12 (PTT)
15                                     4 (GND)
14                                     via 510Ohm to 14,6,15,7

Additionaly connect a chinch cable to the ALC connector of the SunSDR ending in the same DB15 plug (Pin 5 ALC, Pin 4 GND)

Then you need to set the band-data in the „Ext Ctrl“ section of your ExpertSDR software to the correct values (see the chart below).