In a topic on handpan.org 
user G.Spence asked today about “retuning a Free Integral Hang to 440 or 432 Hertz” and if “it actually improve its sound by ‘readjusting’ the harmonics”.
When I visited the steelpan tuner Eckhard Schulz a year ago he took my Free Integral Hang and checked it with his tuning devices. Then his verdict was: “There is nothing correct. They cannot tune in Bern.” There are many who like Eckhard Schulz have difficulties to understand the how and why of PANArt’s free tuning.
I want to draw your attention to the paragraphs in Felix Rohner’s review of Anthony Achong’s standard work “The Secrets of the Steelpan” where he wrote about Achong’s explanation about the importance of the modulations of the partial tones (modes): “This art is primarily about the modulation of the partial tones that makes the steelpan sound spirited and livelily. In absence of this modulation the resulting sound will be dead. The instrument will drain energy from the player even if the sounds have harmonic qualities.”
This may help to understand the physics of free tuning and why “readjusting the harmonics” will never improve the sound but destroy the tuner’s work because there is a physical reason why the modes (harmonics as G.Spence called them) have to be detuned.
Achong emphasizes that detuning is not mistuning. On pp. 862-889 in his book you will find the theoretical basis for PANArt’s free tuning. Achong explains that detuning of the modes is necessary, that it is the main parameter used by the tuner to design the sound dynamics by amplitude and frequency modulation. In fact – as Achong demonstrates – no tuner is tuning really exactly although many of them don’t realise it. If a tuner would tune exactly, his instruments would sound very bad and muffled.
If you have understood Achong’s theory about detuning you will understand why it doesn’t make sense to evaluate the quality of the tuning of a steelpan like instrument by tuning devices and that PANArt’s free tuning is nothing exotic. It is based on decades old practices of Trinidad’s tuners and follows this path just a little step further. Achong’s analysis of detuning draws the attention of tuners and players to the tuning quality beyond harmonic correctness that currently dominates the talks in the handpan community.
There is an explanation in Wikipedia that possibly also helps to understand PANArt’s free tuning.
Readers who understand German will find additional explanations in two articles in the Hanglexikon: Die Schwingungsmoden der Hang-Stimmen and Freie Einstimmung .
Yesterday I found this video showing G.Spence playing his Free Integral Hang: https://www.youtube.com/watch?v=qtxXJXZBemk
I think it can explain why he is not satisfied with the sound of his Hang and why he had the idea that a retuning and “adjusting the harmonics” could improve the instrument (meanwhile he was convinced by the discussion not to retune his Hang, a wise decision in my point of view).
I think the wish to retune the Free Integral Hang has very much to do with the specific playing technique demonstrated in the video. The player uses a technique that increases the velocity of the fingers that strike the tone fields vertically very much in order to transfer energy to it.
This technique works well with a drum skin, but it doesn’t function with the Hang.
The following consideration may possibly explain why: An excited drum skin has a quite large deflection. Therefor increasing this deflection by high velocity of the vertical blow is able to make the sound louder and richer. Compared with a drum skin the deflection of a Hang tone field is minimal. Therefor increasing velocity of a vertical blow has a completely different sound result as with the drum skin: The sound is muffled and distorted. The more you increase the velocity the more chaotic oscillations appear. The metal is stretched and this changes the frequencies of the modes in different ways. As result the instrument sounds flat and mistuned.
A Hang tone field also has a complex internal energy transfer from one mode to the others. Especially in the Free Integral Hang these energy transfer processes are more complex than in older Hang versions. Too high introduced forces influence these subtle processes that produce the character and beauty of the Hang sound in a negative way.
To let a Free Integral Hang sound optimally you need a different way of playing. It sounds a bit paradox but the advise is to decrease velocity and force for a bigger sound. The hands are very near to the surface. The movements are more horizontal than vertical. Not more force but more surface of the exciting finger(s) is used. So the fingers are almost parallel to the tone field. To develop an appropriate way of playing a very calm room is needed. This doesn’t work in a noisy street or in a room with background noise.
The video demonstrates another aspect very important for a rich and pleasant Hang sound. It is how you hold the Hang on your lap. Unlike as in the video the player must close his legs to the position in which the pitch of the Gu (the resonance of the air inside the instrument) is exactly one octave under the pitch of the Ding. When the correct position is reached you will hear the sound becoming wider and deeper. Gu and Ding are now acoustically coupled. Without this integration of Ding and Gu the Free Integral Hang sounds flat and without bass dimension.
If you have developed the appropriate way of playing for the Hang you will find, that the sound is clearer, stronger, richer, more beautiful and you have a wider range to influence it. It also will no longer sound mistuned.