Is it possible the Crack gets quiter?

[mackat]

I've noticed that after a while of it being powered up, I seem to have to turn the volume up a bit more. This is even without listening to music until then, I think. Any ideas, or is it just me?

[Paul Birkeland]

I can't think of anything off the top of my head that would account for a significantly audible difference, but there are sonic changes that occur during warmup, and perhaps this is what you're reacting to.

[Nick Tam]

For one, the tubes need around a full 18-20 seconds to power up so it's probably not a wise idea to plug in your headphones immediately

[Laudanum]

2 things.   On some CD's and LP's, the level is different between tracks.   Same would apply to MP3 or Flac or whatever format, especially between music from playlists where the source of the music is vastly different.  Im not big into computers as a music source and I think there are programs that can equalize the levels,  but not taking those into consideration, this could explain it.
Secondly, our brains get sensitized to a certian level and we can have a tendency to keep wanting to turn it up.   I know you stated that you are noticing the effect without listening beforehand, but I dont quite understand that part of your question/observation.  How would you notice it wasnt as "loud" if you didnt listen beforehand in order to have a reference? 

[Doc B.]

 When tubes are cold they tend to be a little more harsh sounding, then smooth out as they warm up. Are you perhaps hearing more treble energy at first?

[Grainger49]

I was going to say Psycho-acoustics, but in conjunction with what Dan said above, I usually turn my stereo up a little after 5-15 minutes of listening.

[mackat]

Thanks guys, I have a feeling it's a combination of warm-up (I still wait a minute or a few before plugging the 'phones in), different tracks and just getting used to it. Thanks for all the help again!