Still sounds very, very, very like a "scsi voodoo" problem to me, which in effect could be drive or cable related. The termination on the new drive is done right? Tried a different scsi cable? Here's another suggestion: boot without the cd connected, just to eliminate it as a factor. Who knows? Maybe the circuitry in it has died after all these years. I once junked a cd drive in a Quadra that prevented it from booting.

Half of the frustration in working on the 840av, of course, is that it take 30 mins to disassemble and reassemble the thing, especially if you are fiddling with the logic board. I know - I had one with gremlins too, in fact, I had one with several gremlins and almost gave up. I'm glad I didn't, though I do wish it weren't so yellowed.