Gain is a ratio of output power to input power:
Gain (Measured in dB) = 10 x log(PowerOut / PowerIn).
Looking at this definition, in order to achieve a positive gain, PowerIn must be less than PowerOut, but greater than 0. That should be rather intuitive, as otherwise, you will defeat the purpose of amplification.
Suppose we let PowerOut be 20 Joules, and PowerIn be .5 Joules. Our gain would then be: Gain = 10 x log(20J/.5J) = 16.02dB
Now, suppose we let PowerIn be 3.5J. Gain = 10 x log(20J/3.5J) = 7.56dB
Consider the opposite case, if we do not change PowerIn, but only change PowerOut, as in Shayne's case. Again, the same rules of logarithms apply. PowerIn must be less than PowerOut but greater than 0.
First, let PowerIn be fixed at 1 Joule.
Now, let PowerOut = 125J. Gain = 10 x log(125J/1J) = 20.97dB
Next, let PowerOut = 175J. Gain = 10 x log(175J/1J) = 22.43dB
This explains why your volume dial must be increased more to achieve the same SPL. By definition, increasing the output power of an amplifier when maintaining the same input power will result in a lower rate of gain. The output power on your new amplifiers is significantly high. The outputs on your Pioneer may not be at a level that is common to most high end preamplifiers, a condition that is far from optimal, thus causing the seemingly drastic lack of gain when adjusting the volume. This is why you need a lot of knob to give you a little bit of sound increase with the Pioneer / Emo setup.
Additionally, the Onkyo 9555's internal preamplifier is factory optimized to produce the maximum safe level of gain with respect to the amplifier section, hence why a little bit of knob gave you a lot of sound.
Enjoy your new stuff.