I've done some work on sound ports, measuring what happens on a 'test mule' when I opened holes in various places. As usual, drawing parallels between the objective changes that one can measure and the subjective sound of the instrument is not simple or straightforward in all cases, but there are some general trends.
One thing that _will_ happen is that the pitch of the 'main air' resonance will rise when you open a port. The larger the port is, and the farther it is from the main soundhole, the more the pitch will go up. It's the same effect you'd get from making the main soundhole bigger, except the port will usually be more effective, area for area.
The main air' pitch is a function of the size and location of the main hole relative to the volume of air in the box, and also depends rather strongly on how much air the top can move. It's hard to predict, but on most Dreads and Jumbos it's around F# or G on the low E string. Surprisingly, making the body shallower (within reason) doesn't raise it much on a given guitar, but it does 'sharpen' it, in terms of the frequency response curve. With a shallow body and a farly large hole, you may find that you have one note that is noticably stronger than the ones around it. Adding a port is likely to make this effect stronger.
Whether the guitar will sound more or less 'bassy' with a port will depend on about thirty other things, like the way the top works, the location of Mars in the zodiac, and the wholesale price of coffee in Barundi. In other words, who can say?
A port will 'vent' other air resonances in the body, depending on it's location. This will reinforce certain notes. However, my experiment indicated that it will also cause a very small drop in output over the rest of the range. You don't tend to notice the small broadband drop, but you do notice the large gain at specific frequencies, so the guitar might sound 'louder' with the port than without. It is, but overall by only about 1%-2%.
A larger port does not yeild more overall power, but it does change the timbre and balance more. The narrow band gain of power is almost exactly balanced by the wide band loss, but you're 'venting' those high frequency air modes more, so they sing out louder.
I find a small (1") port somewhere near the wide part of the upper bout makes anice 'monitor'; shooting some high frequncy sound out to the player that would otherwise all be 'beamed' out in front. It's small enough and close enough to the 'main' hole not to change the balance and timbre too much, and it doesn't sound much different to the player than in does to the audience. That's the main use I've found for a 'port'.
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