Audio Systems: Frequency response, frequency response curves, frequency response curve

Question
I just got frequency response of my mobile phone speaker. I want improve mp3 ringtone sound created with Audacity, to make it sound better. Here is frequency response curve:
http://img94.imageshack.us/img94/5353/frequencyr.jpg

I just not fully understand the diagram: why Magnitude dB (on Y-axis) have positive values?  Does this mean signal amplification? I just usually saw frequency response curves with Gain/Loss (dB) from 0 to -40 specified in Y-axis.
What means "Magnitude dB re 20.00mPa/V"?

What is low frequency which can be safely cut off in Audacity High-Pass Filter for better music sound on my mobile phone?

Best Regards

Answer
The frequency response curve is relative to the sound pressure level of the testing basis.  The Pa/V is a sound pressure reference level in Pascals per unit volume.  It is not a common reference level, but is more common in Europe than in other parts of the country.

Here are some conversions for reference examples:

http://www.ebbtidepolymers.com/conversion.htm

I think you can apply the high pass filter down to 10Hz if you wanted to, but the speaker of the mobile phone is the limitation for bandwidth if you are planning on using it for a listening device.

As a general rule, even down to 20Hz low end response is considered "high fidelity" for bass quality.   And, I might say that even down to 50Hz catches 99% of the compressed audio stuff such as MP3.

Let me know if this covers your questions.