Both of the replies are correct but not the solution to the problem. The original poster was correct in the last statement he made. The problem is a grounding problem, not power supply problem.
Since the computer was not properly grounded and the MAME cabinet was (where the amplifier was located) you have a huge voltage differential between the output of the soundblaster card and the input of the amplifier. In a best case scenario, the computer is now referenced to the power line NEUTRAL wire which although connects to the GND at the electrical panel, there is enough impedance in the wire to drop at least 5-10 VAC on this wire. So in effect, your soundcard output is not swinging at this 5-10VAC reference. Sure, plug this into the amplifier which is properly grounded and you are effectively plugging in a 5-10VAC, 60Hz signal right into the amplifier. You'll find that once you ground everything properly, it will work fine.
Here are some errors made in the above statements:
1. A 100W amplifier will require an 12V, 8A. This correct provided the amplifier is operating at full load conditions and is a Class D amplifier as most mainstream amplifiers are.
During normal gaming conditions (unless you want to be completely deaf), I would guess youd only be operating at may 10 watts per channel average.
2. What you mean by high-powered transformer supply is actually referred as a linear supply. These are not switching supplies are the benefit is a much cleaner output (much less output ripple). However, for large linear supplies, they are extremely expensive, very heavy, and very bulky. Circuit City, Best Buy, and other audio houses almost always use large switching supplies similar to those used in computer supplies. However, there is much more additional filtering in this case.