Hi, already received your photo. From the looks of it, it is possible to connect your amp to the current setup.
Which channel was your sub connected to? CH1&CH2 or CH3&CH4?
example of operation/setting.
On the HU connection side.
Use the speaker output of your HU(all 4) to connect to the Hi Input on the amp. Please take note the + and -.
Connect the Front Speaker wires to CH1&CH2.
On the right side (CH1&CH2), set the switch to full. Adjust the Level, Bass & Treble accordingly.
Connect the Rear speaker wires to CH3&CH4.
On the left side (CH3&CH4), set the switch to sub. Adjust the Level, Bass, Treble and Sub accordingly. Make sure the other switch is on the 4CH side.
As you might have seen, the hi input actually has 5 pins each side. the middle pin is supposed to be connected to ground.
On the speaker connections side,
Connect your full range/component speakers to CH1&CH2.
Connect your sub to CH3&CH4.
Please take note on the + and - side.
By the way, how many subs are you having? Is the amp bridgeable?
I am not sure how to use RCA(since your HU has SUB preout) and High Input at the same time.
Maybe Bert can help you on this.
Hope this helps too. :)