I also have a Phoenix, and McTrinsic was spot on when he called the board a "diva." It can be so finicky at times it originally made me think it was dead when I got it all the way from Australia.
I removed and reseated chips in the beginning. Not sure about jumpers, but the built-in SCSI was great for me after I got the board to boot. My problem was insanely simple (and annoying!) once I figured it out.
On my board there is a hardware switch that lets you flip the boot sequence between the floppy drive and another external drive. My board only had the floppy, but at some point either during its being shipped half-way around the world, or by me when I installed the board into a donor case the switch had been flipped to External Drive. That tiny change of switch position was all I needed to get mine to finally wake up. Without it, my board acted exactly like how you describe yours. All the lights came on, but it was (I realized later) waiting for a nonexistent drive to deliver the goods. So, it just sat there with a black screen appearing broken.
I worked on that thing for hours.
Then, at a stage where I was about to quit, I simply flipped that switch turned and power on. I was SO close to calling it quits. But then it suddenly sprang to life!
I'm not saying your board has the same problem, and I'd bet you don't have a switch for your board. I do wonder if it is waiting for an external drive to load workbench (probably not but be good to check that off the list!). If you don't have a physical switch, then that would indeed be driven by a jumper.
U70 and U21 aren't populated on my board.
I don't remember if mine had those populated, either. If you want me to check I can pull the case off and see. I could also show you the Agnus I'm running.
I'll see if I can find any photos while I'm having my breakfast in the meantime.