This is an interesting question that needs to be addressed.
First, you need to know that the values you specify at the preset level are offsets or multipliers for the corresponding values at the instrument level. For instance, if you specify 10 for the attenuation at the instrument level and -7 at the preset level, the final value for the attenuation will be 3.
Second, the sf2 specification doesn't allow negative attenuations, that would result in amplifications. This is why Polyphone and other software will not amplify the sound if the final value is negative.
The only thing to do is to amplify the samples or attenuate all other instruments / presets in order to have a good balance between all your instruments.
You can fill a bug report for Viena