XM Radio

I recently purchased XM radio- got the Delphi removable receiver with an FM modulator. I have a brand new Land Rover with a 300 watt Harmon-Kardon that has sounded great until listening to XM. For some reason, on certain songs, I almost lose the singer's voice completely. It sounds like I'm listening to karaoke. On other songs, the singer's voice sounds hollow as though he/she were in an empty gym. I have no idea what's up and the installer thinks I'm crazy. Does anyone know what could cause this? Could it be faulty installation? Could the FM modulator be at fault.

Stereo Installion is not my expert area. Now my friend David, he would know this one. But he's now married with 4 kids, so his time is seriously limited, lol. I'll give it my best shot though. I do know that the inncorrect placement of the antenna can make a big difference with XM. Is the XM antenna inside the car or on the top of it? I know usually it has to be on the top, though there are some cars that can get away with it on the rear deck. Maybe some bad wiring or something, I suppose the modulator could be messing up. But now I am just guessing, so I'll shut up, lol. I hope I helped a little