For the sake of helpfullness, I think that we may separate the differences of the ported speaker designs of myself and Petar. What I have described is the standard, pedistrian model of a vented speaker. Petar's is a hybrid aperiodic-ported design. It is a less-than-vented design tuned to a particular frequency. It's nice and can probably produce a nicer sound than a purely vented design, but in Romy's application (getting somewhat acceptable bass from 4 inch drivers) it may cut the effect of the port to severely.
Let me tell you about the small speakers that I built and had mentioned earlier. They used the Peerless 830854 poly midwoofer with an Fs of about 75 Hz. The goal of the speaker was essentially the same as Romy's...Small with relatively okay bass. After a lot of modifications (including an Oops resonating soundboard back) to the origional design, it was successful. I could play Bach's 2nd Cello Suite on them and enjoy it without the port farting out an indistinct noise. They were very pleasant in the context of non-serious listening. They measure just about 5.5 inches deep, 11 inches high, 7.5 inches wide (roughly) and I powered their 87 dB sensitivity with a barbarian Crown amplifier. Once I have a place at home to set up, I'll be using them again, exactly as I did before...As my "its not a musical instrument but it won't piss me off" speaker.
I am very familiar with the ported M-Audio BX-5a "studio monitor." Perhaps I am dimwitted, but I have no idea how these commercial companies can be so talented at making a port sound like a port!!! Not only does the carbon fiber cone impart such a revolting, idiotic, tone, but the wildly flaired port blows and humms until my ears run with blood! Not even in my days of dumpster diving for free drivers have I gotten such bad port sound as I have heard from nearly all commercial speakers. It may relate to the thin walled plastic or cardboard ports they use. My newer ports are made exactly like your horns, just with a straight profile...Stacked and glued MDF. One difference of the port in my MiniMe speakers is that they are ABS pipe that have been damped and lined with an idea that I ripped off from B&W. Their big thing is dimpling their flaires like the surface of a golf ball. Not to be outdone (I was like that in those days), I spread on epoxy and lined the inside of the port with silica gel balls! The damping effect of this treatment was probably much more beneficial that the aerodymic aspects, however.
Anyway, the best advice I can give is to make your enclosure very versitile. With your drivers, with your amps, with your sources, with your goals, and your music, they are going to need to be very much your speakers...not mine or anyone else's. Building a purely ported design could bring the results I think you are looking for.
As for tuning the ports below the drivers Fs; give it a try... I've heard the "flapping cone" sound, I did some hands-on research. I recently repaired the amplifier of a pair of self-amplified Bose speakers. I pluged the passive speaker into my system and listened to it produce some really screechy, shouting, terrible sound. Once I had the amp fixed, I fired them up to hear the really bad cone flapping drone without the shout or the peaky mid frequencies. Obviously, Better Sound Through Research means using an equilizer to "fix" a fundamentally flawed speaker. So, with the recent findings, I am reconcidering my "impossible to use port's below Fs" attitude.
I'm not interested in having an orchestra sound like itself. I want it to sound like the composer. Leonard Bernstein