Apple Tech Articles
Hopefully the following Apple knowledge base articles will be of some help. The first one is for a symptom of "Screen is Black, No Startup Tone, No LED" (no LED referring to the power LED on the front of the tower). Not sure why, but it doesn't take you into any troubleshooting for the power supply. You can find it here:
http://docs.info.apple.com/article.html?artnum=95155
The second article is for the symptom of "The Computer Will Not Turn On." This article does provide documentation for verifying the power supply and trickle voltage to the logic board. You can find it here:
http://docs.info.apple.com/article.html?artnum=95154
As far as the LEDs go, double-check the labeling. There are a total of 7 - DS1, DS4, DS5, DS6, DS7, DS8 & DS9.
DS7 & DS8 are the most important ones, and should both be on or both off. You didn't mention DS7 in your list - it is located on the left side of the logic board, kind of between the lower end of the hard drive data cable connector and the upper end of the power supply connector.
DS1 inidicates ATA drive activity, and normally shouldn't be on except for a few seconds at startup, and if a hard drive is on the ATA bus it will flicker indicating drive activity. It is located a few inches above DS7, right next to the upper IDE cable connection and to the left of the PCI slot. This should not be on continuously, and if it is it may be an indication of ATA bus problems.
DS4 & DS5 are USB power indicators. On indicates power is being provided to downstream USB ports. They can be on even without USB devices attached, but SHOULD be on if there is a device attached. They are located a couple of inches below the USB port at the back side of the logic board.
DS6 indicates Ultra ATA bus activity. It is located right above DS7.
DS9 indicates that the CPU is requesting the bus. It is not normally as bright as the others, and only lights for short time periods when the CPU has requested the bus. This seems to me to be in line with your note that it is on only when the PM button is pressed.
Finally, you should still hear the startup chime. Lack of hard drive would produce a blinking ?, but you should still get a startup chime and display.
Hope this is of some help. Good luck.