So are we saying the two F hole PRS is a hollowbody and the one F hole is Semi?
Generally hollowbody has more of an acoustic sound and semi provides a nice in between sound with less feedback issues. I believe on the Carvin/Keisel 550 and 575 they removed wood to get the chambered interior but it still has the center block for the advantages it brings.
PRS guitars with 1 F-hole are semi-hollow whilst those with 2 are Hollowbody guitars. The Hollowbody guitars do have a block under the bridge but that is for the bridge to be fitted into. With the 594 Hollowbody, I believe the block is a bit bigger because it has a two piece tune-o-matic type bridge. It makes sense to have something a bit more substantial to screw into and must help with the transfer of tone into the body rather than just the top. Its also less likely to have any problems with the tension on the bridge by the strings as its fixed into the block and not just the 'thin' top.
If you have ever seen the way a bridge is fitted into a solid body, you will know that the post and post mountings are quite long and go quite deep into the body - much longer and deeper than the thickness of the top. It doesn't have a block under the pick-ups at least no more than the neck joint occupies. the rest though is hollow.
The Semi-Hollow is still quite hollow. the 'hollow' occupies the majority of the top part of the guitar and, I believe, the body behind the bridge. It still has the centre block for the bridge and PU's and of course there is a cavity for the electrics too.
What you also need to remember though is that the Semi-hollow is cut into the Mahogany but the Mahogany is also the back of the guitar too where as the hollowbody guitars carve out all of the mahogany completely (apart from the bridge block of course so its basically just the side wall of the guitar and has a maple front and back. The Hollowbody doesn't have back plates either where as the semi hollow does.
The semi-hollow is more like a solid guitar that has weight relief but instead of a lot of small chambers, it has a large chamber at the top and an F-Hole. The Hollowbody is much more like a traditional hollowbody. It doesn't have a centre block as such but it does have a block to mount the bridges and all the wiring is done through the pick-up and F-Holes like a hollowbody should.
I haven't had the opportunity to compare a solid, a semi hollow and hollow version of the same guitar. It is now possible with the 594 as there is a DC version of each. I do have a Solid 594, a Hollowbody ii (both with the same PU's) and a Special 22 Semi-hollow but there is enough variations between all 3 of these that trying to ascertain what, if any, differences there are between all 3 types of body. I also wouldn't trust youtube for demo's of all 3 because all I will be hearing is the compressed audio played through my laptop speakers - not the guitar amp itself - assuming all 3 were in one video (but I don't believe there has been one done yet), you may hear some difference but still won't be like being in the room yourself.
From what I have heard, I think the semi-hollow sounds close enough to the solid version that it may as well be considered more as weight relief. Its like comparing a Solid body Les Paul to the weight relieved Les Paul which, if you don't know which was which, you wouldn't tell on sound alone. However I will say that I have never heard both the semi and solid body 594's played through the same set-up and only ever heard the semi via youtube which, as I said before, is not good enough audio quality to be considered as an accurate representation. So until I have someone who has both, test both with exactly the same set-up and give me an account of their 'in the room' observations.
I can only go on what I have observed and to me, the Semi-hollow doesn't seem to add (or subtract) anything from the sound and seems to be more like a more fancy weight relief with the F-Hole. It makes more difference unplugged of course, more resonant which you can feel playing but as far as the tone coming out of the speaker, the electric recorded sound, I don't hear a difference. Youtube compressed audio may lose any subtle nuances you hear in the room so there could be a more noticeable difference in the room.
The Hollowbody does make more difference to the electric sound coming out of the speaker. Obviously unplugged its the loudest and most resonant of the 3 - its loud enough that you don't need an amp if you are playing at home. It does sound different though which I attribute to the hollow area around the Pick ups and the Maple back. The solid and semi hollow are basically identical between the bridge and the nut, the part of a guitar that the strings are over and around the pick-ups. Both guitars have the same amount of maple and mahogany that the pick-ups are mounted in, the necks are the same, bridge too is mounted to the same build which is why I think the sound is so similar with the 'semi-hollow' and more resonance when played unplugged and that you can feel more when playing - maybe hear a bit more of the acoustic sounds over the amps sound too but in a recording from just the amp (no room mic to pick up any acoustic sound from the guitar), I would have a hard job telling the Solid and Semi apart but the hollow I think would be much easier to detect.
Anyway, that's my thoughts and I hope there is someone here who can provide a bit more evidence of the audible differences between a solid and semi-hollow version. I must admit, I do like the more resonance of a semi-hollow that I can feel against my body but I don't know that it adds a lot to the tone.