Due to a quirk in the SoundFont specification (or at least how it was implemented by E-MU/Creative), 1 dB in the attenuation field only attenuates the sound by 0.4 dB. So, your 144 dB attenuation is only attenuating by 57.6 dB, which is why you still hear sound.
Also, if you're looking to not hear certain zones while editing, you can right-click the zone title in the table view and choose "mute", "activate solo", or "unmute all".
... hmmm, I just noticed that "unmute all" is broken. I will file a bug report about that.
Each parameter has a range. In the case of the attenuation this is between 0 and 144 and there is no way to go outside this (written in the specs). So even if you specify 144 at the instrument level and 144 at the preset level, since 288 it is above 144 it will be back to 144.
As you noticed, the dB are not real dB but for compatibility with all other soundfonts Polyphone also includes this error.
"The volume envelope operates in dB, with the attack peak providing a full scale output, appropriately scaled by the initial volume. The zero value, however, is actually zero gain. The implementation in the EMU8000 provides for 96 dB of amplitude control. When 96 dB of attenuation is reached in the final gain amplifier, an abrupt jump to zero gain (infinite dB of attenuation) occurs. In a 16-bit system, this jump is inaudible."
