I would highly recommend following SSLTech's advice and not touching any more in the unit than you absolutely need to. They are sensitive to diddling around. You're probably right, change at least one of the DRAM's out. U30 is the lowest byte, U45 is the highest. You can swap them to find the culprit based on that information.
The chips in there are relatively old so their ESD protection is not very good and they blow up at the slightest provocation. Follow good ESD precautions in there or you'll end up with a dead PCM70. Also, if you accidentally short out the LM311 comparator in the A-D circuit, you'll take out the CMU chip. Remember that the proprietary Lexicon IC's are essentially unavailable so if you blow it up, game over. You need to ensure the grounding method of any possible replacement comparator is exactly compatible with the original one or you'll hit the CMU with a negative voltage and poof.
The Murata filters are around a 8 or 10 order filter, I don't know of the exact topology but the order is really high. I have replaced them with one of the Linear Tech antialiasing filters along with some additional components - a 78L05/79L05 and a 5532 makeup gain amplifier. You need to etch a little board to do that. They have a 0.5 gain, and they are also very sensitive to messing with.
Yea, Keith may have been a bit harsh without any background but his advice on these is sound. I actually just received a couple of DeltaLabs units that were previously 'fixed' by a tech and returned to the customer as unrepairable. I've got one almost working, I know what chip is not working. The tech replaced the output D-A converter circuitry, where the problem was actually in the input A-D circuit.