Three C cell size batteries per speaker is what you use if you are on the go.http://www.shopping.com/Koss-HD-50/the-paradox-decent-small-computer-speakers/1386310/reviews#tabAnchor
EACH C cell battery produces 1.5 Volts (DC)
3 times 1.5 Volts (DC) = 4.5 Volts
Total of 9 Volts (DC)
Your main concern is with the Voltage.
The Amperage can vary. It can be more than needed.
Because Amperage represents Current.
How much Current is available.
(Water hose example;
Voltage is the speed, or velocity of the water coming out. Amperage is how many gallons per minute that comes out of the hose)
However, you better make sure that EACH speaker receives No more than 4.5 Volts (DC)
Since you do not have the special splitter adapter cord that comes with the power brick, you will have ensure that EACH speaker receives no more than 4.5 volts.
This is if you are trying to use a 9 Volt (DC) power supply. (AC adapter)
Please post additional questions in a Comment.