Subwoofer volume vastly different based on amp. Why?

Summary: I am trying to better understand why my powered subs have very little available volume (i.e. I have the gain dial set to 80-100% gain) when using some amps and a lot of available volume (i.e. I have the gain dial set to 10-30% gain) when using others amps. After doing some research of my own, I think it may be due to the output voltage on the amps, but I would appreciate some confirmation. Below is a breakdown of my experience.
Sub 1 = Monitor Audio R360(Product Page)
Sub 2 = Klipsch RSW-10(Spec Sheet)
- Not used at the same time, but same experience with either sub.- I am only using line-level RCA connections. Both subs have L/R RCA inputs.

Amp 1 = AudioSource Amp100VS using Main Out = lots of volume(Spec Sheet)
Amp 2 = Cambridge Audio CXA61 using Pre-out or Sub-out = lots of volume(Product Page)Amp 3 = Pioneer VSX-824 using Subwoofer Pre-out = not much volume(Spec Sheet)Amp 4 = Rotel A12 using Pre-out = not much volume(Product Page)
Conclusion: Is this difference in volume due to different output voltages on the pre/main/sub outs? The Rotel A12 is the only amp that I can find an output voltage for. They list the pre-out as 1V. If this is indeed the issue, it seems strange to me that more emphasis isn't placed on these voltages. I have seen this article show up over and over again when trying to learn more about this subject, but honestly, I find it hard to follow.

Possible Solutions (assuming output voltage): Use an RCA splitter from the passive pre-amp? Use an active crossover such as the miniDSP 2x4? Other ideas?
Thank you so much for your input.