Top of the line! But in this case, I can only think of a USB interface with DSP like the Tascam 208i or the Motu Ultralite-MK5. With a lot of inputs and outputs that won't be of any help, unfortunately...

Any external equalizer will add latency, nothing to do about it, but you're talking about 2 instances of EQ3, there are 3 AFAIK ? And from what you described, you only need one, don't you ?
Edit: some sound cards are equipped with DSP's, which allows latency-free EQ, what are you using ?

Tout égaliseur externe ajoutera de la latence, rien à faire, mais vous parlez de 2 instances d'EQ3, il y en a 3 sauf erreur ? Et d'après ce que vous avez décrit, vous n'en avez besoin que d'une seule, non ?
Edit: certaines cartes-son sont équipées de DSP's, ce qui permet des EQ sans latence, en utilisez-vous une, si oui, laquelle ?

Vous avez raison il y a bien 3 instances possibles d'Eq3 dans les effets mais beaucoup de presets d'instrument utilisent les 3 instances ( souvent 1 delay et 2 EQ3 ) donc pour ces instruments je dois sacrifier un EQ3 pour le remplacer par mon EQ3 personnalisé. J'utilise 2 DAC ampli casque FX-audio x6 ou Yulong Canary II. Merci de votre aide.

You're right there are 3 possible instances of Eq3 but a lot of instrument presets use all 3 instances (often 1 delay and 2 EQ3) so for these instruments I have to sacrifice an EQ3 to replace it with my custom EQ3.  I use 2 headphones amplifier DAC, a FX-audio x6 and a Yulong Canary II.  Thank you for your help.

Tout égaliseur externe ajoutera de la latence, rien à faire, mais vous parlez de 2 instances d'EQ3, il y en a 3 sauf erreur ? Et d'après ce que vous avez décrit, vous n'en avez besoin que d'une seule, non ?
Edit: certaines cartes-son sont équipées de DSP's, ce qui permet des EQ sans latence, en utilisez-vous une, si oui, laquelle ?

...After somme tests I confirm a too big latence with APO equalizer, and it works only with wasapi (won't work with asio and wasapi exclusive). The addition of another output equalizer (independant of presets) would be a good thing.

what equalizer (equalizer or EQ3) in pianoteq is the more interesting to use

I am under the impression that neither of these work quite the way you want (as a simple fixed or parametric band equalizer).

The "Equalizer" button on the main UI opens a dialog and if you hover over the "?" symbol you will see that it explains that the equalizer is not a traditional one, but affects the instrument model directly and adds compensation as well to keep loudness the same.

The "Equ3" panel in "Effects" is described in the manual as : "The EQ3 effect is a simple 3-band parametric equaliser. Dragging control points on the interface will change the cutoff-frequency and the gain of each band. Pressing the ’shift’ key while dragging will change the Q factor of each band."

So Equ3 is probably close to what you want, but I think not quite what you want as a 3-band parametric seems too limited for your needs.  You can combine multiple Equ3 effects but this seems quite complex to get to do what you want.  You might be able to smooth out the offending HD 599 "bass bump" using it.

This might be worth exploring more on this site and perhaps on the Equalizer APO site to see if a solution to the latency issue can be found.  Maybe you could explain the system setup you use (CPU, what sound API you use and if you use a sound card like a Focusrite or similar) and perhaps people can offer some suggestions.

Not sure how much programming you are will to try but here's a nice site with quite a lot of data, including equalization data for both your headphones.  The curves they give target the Harman curve but they can probably be adapted either by hand or using "some" programming to create your own curve to target the HD 540 rather than a Harman curve.

The site also has quite a bit of information about equalization apps and tools for different OSes (including phones/tablets).

Here's the site : AutoEq on GitHub

And links directly to the two head phones that interest you :

HD 540

HD 599

Hope that helps a bit.

Thank you for your help. I know the link and I tried harman curves for my headphones ( with APO equalizer and peace) but I'm not satisfied with the results. The HD540 without any equalization is the best result for me, so it is the target I want to obtain for my HD599, the APO software works good for this but it seems to add a latence to pianoteq. So my question was what equalizer (equalizer or EQ3) in pianoteq is the more interesting to use.

The site also has quite a bit of information about equalization apps and tools for different OSes (including phones/tablets).

Here's the site : AutoEq on GitHub

And links directly to the two head phones that interest you :

HD 540

HD 599

Hope that helps a bit.

I mainly play with old headphones (Sennheiser H540) which gives me full satisfaction (great linearity and great soundstage).  However lest he let me go one day because of his old age, I took advantage of a promotion to buy an HD599S.  These headphones have a significant bump between 100 and 150hz compared to the HD540, which makes the bass confused in the C1-C2 zone.
To get as close as possible to the sound of my HD540 I have three solutions:
The global equalizer (equalization before sound processing)
An EQ3 equalizer in the effects (output equalization, but some presets already use the 2 EQ3 available)
An external equalizer type APO equalizer (but it seems to me that this method adds an additional latency)
Has anyone studied this problem before or can someone from modartt advise me?
What's the best way to get the best possible result?
Thank you