Soundstage moving forwards

One of the things always intrigued me was how soundstage moves forwards when I play music louder. When playing at very low level, it is deep and somewhat narrow. As I increase the volume, it gradually moves forwards and at 95 dB peaks, it is almost at the plane of the speakers. The depth of the stage also seems flat at that point. What is the explanation? I have standard cone drivers. Is it limited to cone drivers or seen with electrostatic and other types of drivers?
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And also being on the edge of not enough pwr,you should be noticing a smoothing effect on transients.Some people like that,but not me,takes the snap,pop,and immediacy from the music and makes everything sound the same.

 I have tri-amped system with pair of old YBA amplifiers, esp. one driving upper bass/lower midrange (70-700 hz) is the Signature amp with hefty power supply but rated 140 watts at 8 ohms. The range above that driven by YBA 2 delta rated 140 watts at 8 ohms. The sensitivity of the speakers is about 89db/watt. I will try PS Audio Stellar 700 for the midrange and see if it makes any difference.
These are custom built, transmission line design with Cabasse 7” drivers for upper bass/lower midrange and sealed box with Acuton midrange and tweeters. They are in d’Appolito configurations with total of 5 drivers each side with single tweeter in the middle. Subwoofers are separate double 12” Focal with bandpass design. I use Marchand active crossover. All speakers are hard wired with Alphacore Gertz. They are large and heavy!