[JohnM]

On the effect of volume on dips, if you have a true notch (as deep as you can measure) it will not be affected by volume or EQ as it is caused by the reflection having the same level as the direct sound so they cancel perfectly. It is more common for the reflection to be a little lower in level, so the cancellation is partial - if the reflection is 90% of the direct sound that means 10% remains after the direct and reflected sounds mix, so raising the volume or applying EQ lifts that 10% just as it lifts everything else.

The sensitivity to position is fairly typical of small spaces. you may find by moving even further than you have you find a better spot, the RTA view can be useful to quickly try different positions. Treatment at those frequencies needs to be very thick to have any effect, but you might find an angled rigid panel helps to break up the reflections more.