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First up, since you have only one pre output on the HU, the installer has used a splitter rca. What I don't get is why he even bothered to use one if he only connected one pair of input's to the amp. Since you are using both channels A and B, there should have been a second input rca from the splitter connected to the inputs of channel B on the amp. This only means your sub isn't working since the channel it is wired to (B) has no input from the HU.
Also, its better you upgrade the pre and get one that has 2 pre outs with sub control. Cheapest alternative, pioneer 3050ub comes to mind. The problem with running the current setup is, that it only has one pre-out which sends the same signal across both left/right outputs. Since this signal is split up (in your case), any changes made on the hu will effect both the speakers and sub. You won't have independent control over your speakers/sub.
Finally, the wiring you are using is the lowest quality available. Its so bad, it should go in the trash. You might have just started this audio build setup in your car, but when you do upgrade the wiring (power/gnd + RCA's), you will realize what I mean. The wire used for the gnd/pwr is possibly either 8 or 10 gauge (can't be sure) which for a 8" sub and a pair of coaxials is enough.
Also, post a closer picture of the amp setting panel. I want to see what this installer has done. And who even did this install. Grab your keys and head over to haroon radio. I think its in clifton but use the search button and you will find a lot of thread related to them.