Unfortunately, you can't buy extra RS 220 headphones to use with the transmitter/charger base -- Sennheiser doesn't sell RS 220 headphones a la carte. If that's what you need, definitely go for Sennheiser's RS 170 and RS 180 wireless models that let you can use up to four headphones with one transmitter base.
The RS 220 sounds like what it is, an audiophile headphone, but one that just also happens to be wireless. That's notable because of its rarity, and the RS 220 has a degree of refinement/clarity lacking in the less-expensive models. Still, the differences between the headphones aren't dramatic and certainly don't justify a $270 price jump.
The RS 220 definitely unleashes a bigger, more spacious sound field, and the treble is more refined and clear. Classical music CDs sound especially natural, and Leonard Cohen's "Old Ideas" exemplified the RS 220's audiophile appeal.
Turning up the heat with "The Raconteurs: Live in Montreux" 2008 concert Blu-Ray, the RS 220 handled the rock band's massive sonic onslaught with ease. Switching over to the RS 170, the sound showed some signs of strain with the volume turned up loud, but that headphone actually played slightly louder than the RS 220, at the sacrifice of its sonic clarity.
Comparing the RS 220 with another wireless headphone was one thing, but the real test of the RS 220's sound came when I plugged-in my wired Sennheiser HD 580, a close relative to the HD 600 and cited by Sennheiser as a reference for the RS 220 design team. Both headphones sound open and spacious, with the source appearing to come from outside the ear cups instead of sounding stuck inside my head.
The RS 220's tonal balance is a little richer as well, while the HD 580 is more dynamically alive and "crisp," as tested with Eminem's "Live From New York City" DVD. The RS 220's bass is accurate and nicely defined, but probably won't satisfy headphone buyers lusting after heavyweight bass oomph. The wired HD 580 headphone can definitely play a lot louder than the RS 220.
That wasn't a concern with the RS 180, and turning on its ALC (automatic level control) feature boosts the quieter parts of the mix, which is why I recommend the RS 180 for folks with impaired hearing (and it's also a great sounding headphone).
The RS 220 is expensive at $599, and the $329.95 RS 180 sound gets close enough if you don't have an unlimited budget. On the other hand, if you're the type of consumer who wants the best wireless headphone on the planet, the RS 220s are certainly worth it. Other than their inability to play at super loud volumes, they nearly match the sound quality of Sennheiser's reference grade 600 wired headphones, and that's a boon for a wireless headphone at any price.