I have now had a few hours of listening, measuring, and calibrating time, and try as I might, I really cannot hear any difference between the direct path and the PAS 3X path. I also measure no difference using REW and a calibrated mic for frequency response, THD, and impulse response for what that's worth.
I conducted the tests by installing an A/B switch allowing me to conveniently switch between a direct path from my iPhone to the VTA70 and an indirect path going from iPhone to PAS then to VTA70. I first set all the PAS tone controls flat (In the PAS 3X this causes an actual electrical bypass of the tone circuits), Loudness and Scratch filter Off, and balance center. I then took some measurements of pink noise to calibrate the volume of the PAS path to exactly match the direct path.
Then I just sat in the sweet spot and played a variety of music and flipped a hand held A/B switch after blindly turning it around in my hands so I had no idea which path I was listening to at any time. At first I could not tell which was which, and neither one sounded better, but then after a few minutes I noticed the channel balance was slightly off in the PAS. To deal with this I adjusted the balance and volume on the PAS until I measured identical SPL response to pink noice in each channel and continued with the testing.
So I'm not hearing the magic here guys, but I'll tell you what. I LOVE this PAS! Not for improving the sound, but it is really fun. It has a very nice volume control and I actually enjoy the balance and stereo blend function (Allows for easily listening to just one channel with both speakers or mixing to mono for experiments). And it has excellent tone controls! Can't believe so many people bypass them. They are smooth and powerful, and seem to boost just at the right places. Also, the frequency response plots I captured show that when the controls are centered the spectrum looks identical to the direct path, so they really do get out of the circuit. I don't feel I really need them, but it is nice to know they are available if I want. Or if one of my sons comes home and wants to hear bass that shakes the floor.
Also, the PAS makes the overall system a *lot* louder on full volume. The position of it's volume control that causes the output to match the direct path is about 2 O'clock, so anything past this is louder than I could achieve with a direct path. Of course there is potential for overdrive in this region. But you know, spikes and all that transient stuff goes here...
The PAS does add a little noise. A tiny bit of hum, and some rushing hiss. I have to put my ear up to the speaker to hear it, but it's there.
So... I guess some will say it must be a great preamp if I can't tell it's in the circuit.
In any event. I am not saying that preamps can't improve the sound or add special dynamics, just that I can't hear them with the equipment I've got. It is absolutely possible that my equipment (iPhone, PAS 3x, VTA70, Klipsch Heresy III) is just not good enough to show the differences. Though I have heard it said the these speakers are pretty revealing.
The good news is that the PAS actually does not seem to be degrading the sound other than the slight hiss and hum mentioned above. But no new and improved dynamics have appeared in the music. That was nothing but wishful thinking I'm afraid.