Dunlavy SC-IV minus SC-I = a good sub?

I'm going to disagree with the above statements.
I have many years experiences with every model of the original Dunlavy line. I've owned several pairs of the SC-1, including the original prototype speaker that helped to launch Dunlany Audio Labs. I currently have a pair of the SC-3 on hand. And I've owned the SC-5.
I've spent years reverse engineering every model except the SC-6. So I think it's safe to say that I have a fair amount of experience.

My opinion is that it is possible to get extremely good performance from a pair of the SC-1 and a very good subwoofer.
The key has everything to do with HOW you use that subwoofer!
Far too many people try to make it do too much. The SC-1 starts to roll off at 80Hz. And it does so with a fairly slow roll-off towards the bottom end. Your sub should not try to overlap with this too much. I've gotten good results by setting the Sub's roll-off at about 40-50Hz. Most subs will have a 2nd order crossover which means it rolls off faster going upwards than the SC-1 does going downwards. This is partly why people get lumpy bass. They push the Sub too far up the spectrum.
The other aspect to pay very careful attention to is the phase. Most of the time, it works to have the phase on the sub set at 180degrees and have them sit right next to the SC-1. But this is only a generalized starting point. The exact location of the Sub is going to produce a wide range of differences based on it's location and phase setting.
In other words, it's going to take a fair amount of tweaking to get it to work right.
But the truth of the matter is that you CAN get extremely good performance with an SC-1/Sub combination.

I also used to sell Dunlavy back in the day. And one of the guys I worked with had been an employee of Dunlavy from their start. We sold quite a few SC-1/Sub combinations with great success.

Bottom line, the SC-3/SC-4/SC-5 etc. are an ideal way to go from a technical design standpoint. But in real world situations and real rooms and real practicalities, sometimes it's actually better to go with the SC-1/Sub combo.
I have a friend who, not long ago, switched from using his SC-3 to a pair of SC-1 with a pair of Subs. It took quite a while to perfect the set-up. But there is no doubt that in his room, the current set-up is superior.

If you want a good Sub suggestion, take a look at Vandersteen first. Theirs is one of the very few that use a 1st order roll-off for the Sub. This will make things many times easier when trying to set them up. You won't have to worry about phase as much. And the only real tweaking will be finding the same relative distance to your ears from the sub's output as that of the SC-1.

All in all, there is one significant disadvantage to this approach. Because there is no crossover controlling the bottom end roll-off of the SC-1, it won't play quite as loud as the larger units with the same dynamic linearity. Even though it has the subs to fill in the bottom two octaves, there is nothing stopping the midranges from trying to reproduce those frequencies. So at very loud listening levels, or on music with massive amounts of bass, there will be an increase in IM distortion.
But the whole point of using this set-up is to help accommodate rooms where the SC-3 and bigger models end up having too much bass problems. In such a room, the relative volume will usually be lower anyway.

Hope this helps!

Prdprez, based on your experience, which modal is the best sounding Dunlavy seakers?

Thanks!

My experience (as is most others familiar with the line) is that they all sound pretty much exactly the same with the exception of bass extension and linearity in larger rooms.

Though, were I to pick just one, it would be the SC-5.
That one was the only one to use a slightly smaller and more dynamic midrange cone. To my ear, this meant better midrange clarity and tone. But it still retained the ability to play 20Hz with power, even in large rooms.

Case in point..... In my party days I used to come home from the clubs and crank up some 20Hz sine waves late at night. It was juvenile fun hearing my apartment neighbors talk about the minor "earthquake" we experience the night before.

From searching on the internet, I do realized that some of the SC-5 I saw used dome mid and some used cone mid.

Do you know the story behind the change? Which came first, the dome or the cone?

The Dome came first. But it didn't last long in production.
The domes had wonderful measurements. Absolutely superior impulse response. But often failed in the field, many times due to nothing more than shipping.
The cones soon replaced them and didn't change after that.