I had similar problem on a 750Gb xxRBA0.
I initially swapped the heads and first of all it seemed to be OK, reading from some heads perfectly, then it stopped. (I had to use smart hot-swap to get the drive to read sectors, as it seemed that the heads were compatible enough to read the SA but not quite good enough to allow proper translator initialisation)
I checked the f/w several times, all mods checked out OK. Drive passed all head tests OK, but no further LBA access. Also checked the PCB, it was fine.
So I put some more new heads in... voila! LBA access again

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