you did not realize this at the time but you were ahead of the game when you had that eureka moment listening to the SC IVA speakers. The sonics from that speaker had less to do with the drivers being acoustic suspension & had a lot to do with their using 1st-order x-overs (like rcprince wrote) & the fact that John Dunlavy designed them to be time-coherent. I've seen time-coherent speakers to be ported & sealed box with the sealed box speaker giving a slightly better quality of bass. Careful design of a ported time-coherent speaker can yield very good bass as well.
In the S'phile review link you posted if you look at Figure 8 in the measurements JA took you can see step response of the SC IVA that clearly shows the time-coherency of the speakers - the sound waves from all the drivers reach the listener's ears at the same time - there is just one rising edge. In speakers that are not time-coherent you will see several rising edges as the various drivers sound waves reach the listener (look at the step response of any B&W speaker in S'phile's reviews).
if you wanted to repeat your eureka moment again today you would need to buy another time-coherent speaker. Unfortunately Dunlavy speakers are no longer a choice but you do have several other choices - Green Mountain Audio, Eminent Technlogies, Sound Lab ESL, some older Quads (maybe even incl the original ESL 63), Vandersteen, older Thiels made when JT was still living.
The use of a 1st order x-over in time-coherent speakers is not a coincidence - the math shows that a 1st order filter is the only filter that will preserve the phase relationship between any 2 signals over the entire 20Hz-20KHz region. All other higher order x-overs can only maintain the phase relationship at the x-over frequency & over a very small freq range above & below the x-over frequency. That makes speakers using higher order x-overs only phase coherent (which is quite easy to do for most speaker designers) but it is far from sufficient to give the listener an eureka moment when listening to music. I've been harping about time-coherent speakers for quite a while & how they are the only way to go if one wants to listen to music with a realistic sound over the long-term i.e. get off the speaker merry-go-round. Fortunately for me a few Audiogon members converted their existing speakers to time-coherent using software (DEQX) & they have reaped tremendous benefits from doing so. They couldnt be happier....
it would be great if you could read thru the following threads so that you can educate yourself about time-coherency & its high importance to speaker design & music playback. Sometimes these threads ramble on - needless to say skip over those sections & concentrate on the important posts esp. those by Roy Johnson (of Green Mountain Audio):https://forum.audiogon.com/discussions/is-deqx-a-game-changer?highlight=is%2Bdeqx%2Ba%2Bgame%2Bchanger https://forum.audiogon.com/discussions/sloped-baffle?highlight=sloped%2Bbaffle https://forum.audiogon.com/discussions/time-coherence-how-important-and-what-speakers
(this, i think, is the best thread ever in Audiogon. it's really long but there are many details & technical discussions re. time coherent speakers that are very enlightening. Try to comprehend & assimilate the info in this thread. if you want to read just one thread, this would be it).