Reading your first post again (now understanding that the subs are passive and that the "preamp" is really an amp), it seems like you have it hooked up correctly. There are two possibilities:
1. The AVR sub settings are not correct - as others have said, the main speakers should be set to small and the subwoofer level should be cranked up to make sure it's not just operating at a very low level, sub crossover should be 100hz or higher.
2. The gain on your subwoofer amp is not sufficient. Does that amp have a volume control and is it set to max? You have an amp and passive subs that don't go together, which is absolutely fine, but it means that you may need to artificially boost the gain somewhere in the signal path to the sub amp in order to get normal subwoofer output levels.
It would be a lot easier to help if you specify the brands and models of the receiver and the sub amp, and also the settings within the receiver (speaker size setting, subwoofer level, subwoofer crossover, etc).
PS: It looks like the receiver "Zone 2" output can be a preamp output for the receiver, so as a test, you could connect the sub amp to those receiver outputs to eliminate the whole subwoofer setting variable on the amp. If that works, then you know you have the Sub/LFE settings wrong. If it doesn't work, the you probably don't have enough gain in the sub amp (or the amp is broken).
PS2: You could easily test the sub amp by connecting the speaker outputs to your main speakers and driving the sub amp with preamp out.