If I set the EQ to +1db for all frequencies, is that the same as setting volume to +1db?

Yes.

Does Loudness boost all freqs by the same amount, or just low freqs? By how much?

This one's a bit more complicated. Loudness boosts low frequencies more than high frequencies, but the amount of boost is also determined by the volume control. If you have your volume control set to 0dB, the Loudness is not affected. The lower your volume control setting, the more frequency shift you get from Loudness -- but since the amount of Loudness is inversely proportional to volume, what the actual audible effect of Loudness change is becomes subtle and difficult to define.

tanstaafl.