Volume control before amplifier? Pot range...?

Building a powered sub woofer. Enclosure and mounting comes along fine, in 2 days I'll get a 100 W RMS Velleman kit. I will only connect it to the sound card, and sometimes to an MP3 player, in which case I will split the signal in 1 or 2 stereo plus one bass channel. I have noticed very different volume levels in the bass register of various recordings, sometimes even damaging (to my other, portable bike sub woofer).  The new, yet to build amp is even more overrated in respect to all the other, separately amplified sound system components---  So I need a volume control potmeter before the amp.

What is the range this potentiometer should have? (right now, I plug it in the CD RCA input of another amp)

Try something like 10K logarithmic track pot.
. When you go looking for your potentiometer, logarithmic track is often referred to as audio taper
Yup and yup again. BTW, that's called an "input attentuator".