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Wow, ya get off the list for a few days...
I did a rather long experiment with ports a couple of years ago, and there are some pics on my web site of the experimental guitar. Let's see if I can summarize in in only a few chapters...
There are two kinds of air resonances on guitars, which you could call 'cavity' resonances, and 'pipe' resonances. Cavity. or 'Helmholtz', resonances involve a 'bottle' of some kind that is pumping air in and ou from the 'neck'. This is what you hear when you blow across the top of a soda bottle. This type of resonance needs a hole.
Pipe resonances involve air 'sloshing' along the length of a 'pipe'. If you get the water sloshing along the length of the bathtub, that's the same thing. These don't need a hole, but if there is a hole in a place where the pressure changes a lot (like at the end of the pipe) you'll hear it. The hole will also change the resonance frequency, and, because it's radiating energy from the resonance, make it weaker inside the pipe.
What we usually call the 'main air' resonance on the guitar is a cavity or Helmholtz type of resonance. It's the lowest resonanace on the guitar, and has a lot to do with the low-end timbre. It's frequency is set by the relationship between the soundhole size and the volume of the box, and also somewhat by the location of the hole. A larger hole makes the frequency higher, moving the hole up toward the neck makes the frequency lower. Any time you add a hole anywhere else it's like making the main hole bigger. The farther from the main soundhole the 'port' is, the more effect it will have on the frequency of the 'main air' resonance. Thus you usually want to make the main hole smaller if you're going to add a port, but it's hard to predict just how much.
There are a lot of pipe-type resonances in the guitar, with the air 'sloshing' along the length of the body, and across the bouts, and making some pretty odd patterns sometimes. Some of these pipe modes are 'heard' by the normal soundhole, but most of them aren't. For example, the air 'sloshes' across the lower bouts of the guitar at around 600 Hz, but you can't hear that on a normal flat top because there is no pressure change at the normal soundhole location (you do hear it on archtops, one of the differences...). Most of the energy that finds its way into sloshing that air gets wasted. A hole in the wide part of the lower bout on one side will let some of that sound out, and make the guitar a lot louder at that frequency. Thus it will change the timbre. Once again, a hole almost any place in the sides will 'hear' something that you don't normally hear, so it will tend to make the guitar sound 'different'. Better? who can say?
Some of the air sloshing resonances can work with the wood to produce sounds. This is usually true of the lengthwise pipe mode, at around 350 Hz. It will work with one of the top resonances (sometimes called the 'long dipole mode'), so that the top 'extracts' some of the energy from the air that you would not normally hear. This sort of 'coupling' tends to make the sound more powerful and more 'interesting'. Putting a port up by the base of the neck, or in the tail block, will enable you to 'hear' this mode more directly, and will also raise it's pitch. In that case, it might not work as well with the top (or it might work better). For sure a hole in the end of the guitar will make tat air mode weaker inside the box, so no matter what sort of dance the air and the top are doing or not doing, it will be less active with a port in the end.
A port does extract energy from air resonances you would not normally hear, but it's not a free lunch. Most of the energy that moves the back comes from pressure changes in the body, and the port cuts those down over the entire range of frequencies. Thus you tend to get a relatively large increase in sound output at one particular frequency, but a small drop in output over the rest of the range. The two seem to nearly balance out for almost any reasonable size of port, so that the overall effect on the power output is to add 1-2% over the whole frequency range. A larger port doesn't make the sound any louder, it just makes it more 'different'.
On 'normal' guitars most of the sound comes off the top and out the hole (that's how you hear most of what the back does). The low frequency stuff goes in all directions, but the high frequencies tend to be more and more directional as you go up. The player only hears reflections, if they're lucky. A port in the bass side of the upper bout will direct some high frequency toward the player, and makes a nice 'monitor'.
Aside from the exprimental guitar I've made two with ports. Both were Classicals with 1" diameter holes just above the wide part of the upper bout, where it will point toward the player. On one I put a nice surround to round over the edge of the hole, but the other just has a hole... I liked the surround, and am going to keep working on the idea: it looked nicer even if it didn't work all that much differently. I reinforced both of them on the inside with a piece of 1/32" birch plywood with the face grain crosswise, epoxied in place.
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It just makes the engineering side of my brain buss with curiosity like a cat in a hampster hutch. 