I have installed the following in my car for audio thru Blitzsafe adapter:

iPod
XM Commander
DVD/MP3 Player (still need the nav-hack)

All 3 devices are Y connected into the audio adapter to the OEM stereo. I have noticed when I play only the XM, sounds great. When I power on (not play) either the iPod or DVD/MP3 player, the XM audio output cuts down to 50%. Sounds like I need some sort of audio amp to help push the XM sound up but why is it just the XM? Has anyone run into this and if so what was your fix?

Here is how my audio cabling is setup (follow if you can):
From armrest, iPod is connected to a Belkin power/sound adapter. A 3.5mm phono plugs into the Belkin and other end of the cable to RCA port I installed. The 3' RCA cable is then Y connected to the Blitzsafe adapter. The other end of the Y adapter is a pair of 15' RCA cable running to the trunk. In the trunk, the XM commander is Y connected to this 15' RCA cable. The other end of the trunk Y connecter is another 15' A/V RCA cable from the DVD/MP3 player in the front. So, the DVD/MP3 and XM audio together travels back to the Blitzsafe adapter on the first 15' RCA cable. Follow? That is my audio setup.

 

As a general rule, running multiple sources into one input with a "Y" adapter is only OK if only one source is on at a time.

Having problems when more than one device is on at a time is not surprising, and not something that an audio amp would fix.

You need to have only one device on at a time, or you need a switcher that isolates the various devices. Soundgate is introducing one (http://www.discountcarstereo.com/detail.aspx?ID=612) and PAC is supposed to have one, but I have heard bad rumours about the PAC. The Peripheral SSC2 only works with 2 sources.