The coliseum bass amp originally came with a three prong power cord... check the schematic of the power supply currently posted on Rich Briere's site. I am assuming that whoever did the splicing spliced all three wires properly.
I guess the answer lies in what you plan to do with the amp. If it will be set up for practice and never moved, then you should be ok. If you plan to gig with it and move it around a lot, then chances are pretty good you might pull the splice apart.
Personally, I would replace the entire cord. The "type" isn't all that important... it should be heavy enough to carry the current, definitely a three prong version, and the length is up to you. K-mart has three prong replacements and they work out fine. If you really need length, pick up a three prong extention cord of your choice and cut off the female end.
Replacement is pretty straight forward. Unsolder the three connection points inside the chassis and solder the new cord back the same way (i.e. black to black, white to white, green to green). To make it easier to get through the grommet, I would stick with the original thickness of the cord, but if you want thicker you can always open the cabinet hold and replace the grommet.
Do replace the grommet or your cord will eventually be sliced.
