From a raw construction perspective, a block made up of stacked plates will lose a fraction of inter-block transmission as the wave crosses material boundaries. The tighter you couple the plates, of course, the lower the mechanical impedence and so the lower the amount of loss. So you might notice a bit less sustain in a stacked block. But it'd still be much more sustain than you'd get with no block at all.
Where the stacked plate version becomes audibly problematic is where the individual plates aren't exactly matched for volume and mass. Each individual plate therefore provides a slightly different superharmonic resonant structure. These can, under perfect conditions, create sum-and-difference resonances in the audio band that can affect the tonal qualities of the instrument.
The difference isn't huge, and in most cases virtually inaudible.
From a practical standpoint, if you're set up to cut brass plates, odds are you're set up to cut blocks as well. So there's no advantage to the manufacturer to go with a stacked plate design, when it multiplies the tooling time.
Now, as Bob notes, the block-to-body contact plane is important. You do want that to be as tight a fit as is practical for maintenance purposes. Again, this is a function of the mechanical impedence of the joint. Fortunately, here you're dealing with one material that has a bit of compliance, in that you can compression-fit the block into the cavity to achieve makimum contact and thus minimum impedence. The downside of this technique is that it makes it virtually impossible to get the block out later.
Alembic doesn't go all the way to an interference fit, but they do get a reasonably snug slip fit that, with proper tooling, can be disassembled. And, in general, the coupling they lose with the slip fit, they largely regain via the attachment screw that holds the block down.
Oddly enough, one of the weakest points in the mechano-acoustic chain from the string to the block is in the two bolts that hold the bridge on. By necessity, they're stainless steel, so there's a material dis-similarity at two boundary points. Plus, the screws have a proportionately narrow cross-section, and so present a relatively high mechanical impedence to the wave. This coupling has a greater affect on the overall tonality than might otherwise be thought, and is certainly a component in the Alembic sound.
nic
