I'm sure anything would work. My concern is about the sound quality of
whatever circuitry I'm introducing into the signal path between the
soundcard and the headphones - both of which are clean/high quality
(Audiophile 2496 and Sony SU-V6). I don't want to introduce any
distortion beyond what is there already. Would using, for example, a
really cheap $15 set of desktop speakers with a headphone jack introduce
distortion to the signal? Or is the distortion in those cheap speakers
coming from the speakers themselves and not the amplifier circuit
connected to the headphones jack?

I'm considering getting one of those powered speaker systems like
Creative Inspire T3000 $50 or more the expensive Logitech Z-2300 $150.
Because they have nice remote (wired) volume controls with headphone
jacks that you can place right next to your keyboard or where ever it's
handy.

Another option would be a small headphone mixer like from Artcessories
or Rolls. Anyone use those? I only need a single headphone out.

Or just getting a smaller, more compact stereo integrated amp than the
one I'm currently using for this purpose which is a full-size home audio
Technics unit which is taking up valuable space on my desk.

Any suggestions?