W140 Radio Head Unit Change

[KenP]

OK guys, I've done a search and read the numerous posts about changing the head unit in a W140. Some talk about ripping everything out and replacing with new amps, speakers, etc.
Others mention changing unit and bypassing amps, etc. Some mention running extra leads to the antenna circuit to turn on the amps.
Confusing.
I've got a 1996 S500 Coupe with the factory Bose 11 speaker system. I like the way it sounds so don't want to change the install other than the head unit.
The original head unit, of course, is AM/FM/Cassette. (M-B, what were you thinking???) I want to replace with an new Alpine AM/FM/CD/MP3 unit that's XM ready.
According to the Soundgate web site, all that's required is to use their OEM4A interface box between the new head unit and the factory wiring harness that's already in the dash.
Has anyone done this? Is that all that's required?
Thanks, KenP