I have the Grace 901 headphone amp which includes balanced inputs, but it's beyond your budget. I think there are a few Headroom amps that include balanced inputs, but same price issue. I wouldn't worry too much about making sure it has balanced inputs.
Also, there are many amps available in your price range. I would go to Head-fi.org for suggestions. If you can swing it, the new Emmeline XP-7 headphone amp is supposed to be great ($495, also available with a power supply for $695).
Also, there are many amps available in your price range. I would go to Head-fi.org for suggestions. If you can swing it, the new Emmeline XP-7 headphone amp is supposed to be great ($495, also available with a power supply for $695).