typical sensitivity of powered subs?

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I just bought a new Onkyo TX-SR504E surround receiver mainly as a playground amp for a PC crossover (Thuneau Frequency Allocator). As a very first step (not PC related yet), I tried to feed a stereo signal to two satellites and a sub (a Vifa M25WO49 powered by an old stereo amp). I connected the main speakers to front left and front right which I defined as small, set a crossover frequency of 120 Hz and disabled all other outputs except for the subwoofer.

From what I gathered from the manual and service manual, the analog stereo signal is A/D converted, filtered and then D/A converted and output into three channels (high pass filtered L and R, and low pass filtered mono sub).

This seems to work ok (sub signal is output in stereo play mode, but not in direct or pure audio play mode). However, the sub out preamp signal seems much too weak. I needed to set the volume on the auxiliary amp to about 50% to obtain normal output on the subwoofer, wheras when feeding a normal line level CD player signal to the amp and sub about 10% gave me more than enough volume. I changed the relative volume of the sub in the receiver setup to + 12 dB but still needed about 30% on the auxiliary amp.

A look at the schematic showed me that the preamp out sub signal is amplified 10x (+20 dB) relative to the other seven signals that go directly to the power stages that have a gain of 28x (+29 dB).


So my question is:
Is it normal for commercially available powered subs to be increadibly sensitive compared to normal power amps? Or should I return the receiver for repair?