Audiophiles who are less than thrilled with the sound that they get from their iPhone 4 or 4S now have a new companion accessory that is bound to make a difference in the quality of their listening experience. The US$650 V-MODA VAMP, available today from V-MODA.com and Amazon.com, is a rather different iPhone 4/4S case that packs a lot of power and sophistication in to a solid metal casing.

This isn't just a battery case, although it packs a 2200 mAh battery. It uses an audiophile-grade digital-to-analog converter (DAC) to extract the digital audio signal of the iPhone, and then runs it through a two-channel 150 mW amplifier (five times more powerful than that built into the iPhone) to bump up the signal.

The VAMP has two audio processing modes built-in: the PURE mode processes audio to provide the truest sounding audio playback, while a VQ mode adds a "slight spatial 3D soundstage, tightened bass, and increased treble attack" to punch up the sound.

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All of this is integrated into a brushed aluminum case with red highlights. The area that cradles the iPhone 4/4S is made of a silicone material for easy insertion and removal of the phone, as well as protecting it from bumps.

Probably the most unique feature of the VAMP is the built-in optical audio output. This allows audiophiles to connect the VAMP to an A/V receiver or DJ mixer over Toslink or S/PDIF optical audio. As V-MODA notes, once your phone has been retired from your pocket, you can still use it as a high-grade audio source.

The company is donating $50 of the sale of every VAMP to the InTheLoop Foundation, a non-profit that exists to educate and promote "safe sound." As expected with an accessory that costs more than most iPhones, the VAMP also comes with a one-year warranty.

I had an opportunity to try the VAMP prior to