Well you can use a 9 volt in a halo, but it runs out in about 280 balls and swicthes off.
And too add to QP's point on resistance lets get techy and look at capacity for Varta batteries
Type Voltage Capacity Weight
/V /mAh /g approx.
AAA 1.5 1200 11
AA 1.5 2600 23
C 1.5 7800 61
D 1.5 16500 134
PP3 9.0 550 46
As you can see a 9v battery has only 21% the capacity of an AA,
2 9 volts weigh 92 grams, 6 AA's weigh 138 grams, and 6 AA's are far cheaper than 5 9volts which you would need to run the Halo (approximately) for the same time!
So 9 volts just arn't worth it, they are a crappy battery at the best of times!
Laterz
Ian