Room gain, again

Absolutely, it's all about your HT space, and it's longest dimension.

If you could measure outdoor first, you could determine a great deal more. But typically you can see tendencies via measurments, even indoors.

Brief re-cap;
Oftentimes room gain encompasses both Boundary Gain, and Pressure Vessel Gain (PVG). Pressure Vessel Gain (PVG) is whereby the longest dimension of the room can no longer support full propagation of the waveform. At this point, the acoustic propagation transitions to acoustic pressurization. The manner in which the sound is reproduced into the space changes from a normal cyclic propagation, to pressurization because the wavelengths are too big for the space.

The frequency at which this occurs is approximately the point whereby half the wavelength of a given frequency is equal to the rooms longest dimension. So, a 20 hz frequency has a wavelength 56.5 feet. So half of that, 28.25 feet, is the point of transition. Any frequency below that point pressurizes the room, and any frequency above that point propagates freely. So in this room that's approximately 28 feet in the longest dimension, from 20 hz downward, the room gives back acoustically.

At this frequency, the results are a gain in acoustic pressures in the room that grows as the frequency decreases. This acoustic support reciprocity, is theoretically 12db per octave. The percentage of the 12 db/octave gain one achieves, entirely depends on the integrity of the boundary walls and surfaces. If it was the theoretical concrete bunker, a full 12db/octave boost would occur. Typically, somewhere between 6-10 db octave could result. Also, in addition to the walls and surfaces flexing, other aspects may affect the point at which room gain begins. Furniture, cabinets etc, anything that consumes a certain measure of cubic feet, may slightly alter the transition frequency merely because the items take up space.

What's your longest dimension in room?

Have you examined the time domain of the LF in your room?

Thanks