Some of you have neglected to include information about your setup within your user profile, for forum members to better assist you whenever you encounter problems.  So, nobody may take you seriously.

Without problems, you should reach volume levels similar to those of the demos on a Pianoteq instrument product page, if your configuration is a non-issue problem-wise.  Although, demos were likely normalized to sound a bit louder than your installed Pianoteq standalone version.

Maybe you'll want to load a comparable MIDI file or two (2) into your standalone copy for some comparisons, basically, against a few of the demos of unmodified presets on the page.  And, check volumes that way!

Personally, if I were a newbie to this forum, I'd wait a considerable length of time before I attempt to help any others.  My personal experience is; I waited about seven (7) years after I began using Pianoteq  —and then joined in forum discussions.  Although, I always read posts throughout the period.  Certainly, my situation might had differed, had I encountered problems and needed help to troubleshoot!

Now I'm saying, let a process work and allow experienced members to help with new members' concerns.  Otherwise, just as this thread went unresolved about a month before revived, you'll just have the blind leading the blind.

I guess the problem is somehow related to the overall sound configuration, perhaps even the soundchips on motherboards or soundcards in use ...

I'm not exactly sure that I understand what you are saying. I thought originally that you meant you could not hear Pianoteq in you headphones very loud while monitoring it.  Are you saying that the files that you output (i.e. wav files) is not very loud?  If that is what you are saying and you are only recording Pianoteq and not multi-tracking other instruments with it, then while recording shoot for something a little higher than the normal of -12 db, perhaps -6 db on your loudest part.  Then when you go to render your final file that you will embed in video or standalone file, use a limiter on your master channel to raise the volume to your liking, without distorting or losing too much dynamics.

Thanks for a reply. I guess it should be at a normal volume level while I am playing the piano using Pianoteq only without a DAW.  But it isn't. I can assume there is a problem in the VST, not DAW. It is like the loudness is 2 times lower then it should be. Other methods (limiters, expanders and etc) don' t solve the problem unfortunately, If a Iad to multytrack, the other instruments would blow up my house

There could be several possibilities that is causing this issue.  By your wording, I am assuming that you are using an internal sound card on a computer instead of an audio interface?  If so, what is your speaker volume set at (overall speaker volume for your computer)?  You actually have that volume control and you have the volume control in PianoTeq itself.

Other things to check.  Look at the MIDI monitor and see what velocity you are receiving from your controller keyboard, could be a problem there and you're not receiving a high enough velocity via MIDI.

There could be an ohms mismatch in the headphones that you have and the computer.  Some headphones require significantly more power out than others. 

The more information you can provide about your system (OS, Sound Card/Audio Interface), controller, headphones, etc... will help in helping you.

I actually have an external sound card. There is not a problem with the headphones  because I can listen to another songs (not pianoteq) with the perfect volume, and that is not about velocity, I wrote perviously, that I can see the same problem in others' pianoteq tracks, I am not so simple not to realise that the piano sends low velocity notes to the pianoteq. Windows 7 Focusrite ASIO Ableton

Other things to check.  Look at the MIDI monitor and see what velocity you are receiving from your controller keyboard, could be a problem there and you're not receiving a high enough velocity via MIDI.

There could be an ohms mismatch in the headphones that you have and the computer.  Some headphones require significantly more power out than others. 

The more information you can provide about your system (OS, Sound Card/Audio Interface), controller, headphones, etc... will help in helping you.