NOTTNICK wrote:
I need to put a variable pot in a line signal circuit.
I guessed at 10 ohm.
I'd appreciate knowing if this was the right choice.
Nick

Unlikely. This http://www.google.com/search?q=define:rheostat
is probably the wrong way:
(...and "variable pot" is redundant.)

NOTTNICK wrote:
I use my PC for music playback.
I need to be able to adjust the line level output from a USB sound card
from zero to maximum (ie a volume control)
before plugging it into active loudspeakers (with no volume control).

Some concepts you need to get a handle on:
http://www.google.com/search?q=define:Potentiometer
http://www.google.com/search?q=define:voltage+divider
http://www.google.com/images?q=volta...+Potentiometer

.....and something you hadn't even considered:
http://www.google.com/search?q=Poten...Audio+question