I have sent in a headphone amp. I got offered to lower the gain some to get more use of the volume knob. But is there all there is to change gain... Lowering gain gives lower max volume. It doesn´t effect current output or voltage swing at the same SPL level or anything else?

99.99% of amplifiers have a volume control (actually an attenuator) at the input. This attenuated signal is then passed to an amplifier stage that operates at a pre-set amplification level (called 'gain'). Nearly every amp that I have experienced has far too much gain, hence the volume control (attenuator) must be turned right down.
 
There is more than one way to reduce the maximum output. One way (the amateur way) is to divide the voltage immediately before or after the attenuator so that far less is passed to the amplifier stage. This does no favours to the signal-to-noise ratio (...but you might not hear it anyway). The better way is to design the amp section with lower gain from the outset. Or to reduce the gain of the amp stage if it already exists. However doing this requires good knowledge of the amp design, because some circuits do not like to run at low gains (for example, some op-amps are not automatically stable at low gains). Finally the output can be reduced by introducing a resistance between the ouput and your 'phones. This works as a simple voltage divider, and obliterates any 'damping factor' that you may have had (which may or may not be of importance or relevance, depending upon your 'phones, etc.)
 
So, back to your question... if the reduction is by way of amp stage gain modification, it is likely that A) the ouptut current potential will be unchanged, and B) the peak potential voltage swing will be unchanged. However any competent engineer doing the work should be able to advise you of the facts pertaining to their intended work.
 
Cheers for now,
L.

Thanks a lot just what I hoped for. I will take this forward to Arn .