Bass Response on Dunlavy SC-IVa

Try this: Move your listening chair,place a ladder where your chair use to be. Play a constant test tone at about 60hz. Climb the ladder up to ceiling height, make reference of the 60hz tone with either an spl meter or your audio memory. As you come down the ladder at each step make another measurement and record the output, do this until you reach about 2 feet from the floor. Now after you have done this move the ladder, re-install your chair at the original position and take the same measurement at ear level {38to40in.}..At ear level you will have a 6 to 8 db suckout..in contrast to the floor and ceiling measurements.The frequency of the suckout maybe 50 to 80 hz so your reference tone may vary slightly.While doing any of these tests do not touch the volume control. ..You can mitigate some of the phase cancelation by moving your ceiling or by building an acoustic lens attached to the ceiling ..Tom

I agree with Theaudiotweak. But a potentially easier way to mitigate phase cancelation here might be to just lay the IVa's on their sides. But that solution is not without it's own problems. :)

John, Peter Snell had a speaker [Type 1} that had its drivers near the floor level on an angled baffle tilted back with a ramp in front to reduce the floor bounce effect..He also had a U.S.patent on baffle designs to reduce room boundary effects. Truly a designer who thought outside of the box. Had a in home demo of that Type 1 speaker and a sales meeting with Peter Snell...that was probably in 1980 or 81..to bad he is no longer with us.He was truly a innovator. Tom

It is a very interesting question, cathedral ceiling and Dunlavy 4.
I had never known it until I bought a Behringer deq2496. It also took me a week to spot root cause of the problem. However, I got a 20dB hump, instead of suckout, from 45 to 65Hz at listening position, 3.5' from floor. If I move vertically up to 7' from floor, the hump reduces significantly by more than 10 dB.

Yeah you're dealing with differnt issues with speakers designed like that.
The basic "layman's" answer to all of this is that the IV's will do better set up in large spaces, or where at the very least they're on the "Long wall"!(where you can get the speakers spaced further apart!
Yes these speakers have the potential(set up right) to have very smooth, fast, well balanced bass with the Dual woofer Dappolito design(not to mention 1st order cross, multiple driver array). One woofer might balance out the other on the speaker, placed on a "different plane" in space. Still, in small room settings, or where they're not more "optimally set up", you'll have comb filtering issues, boundary to woofer cancelation/reinforcement issues(compounded with multiple woofs on the same planes), and other compouned issues. Sitting at "tweeter level" with your ears is also more critical in regards to comb filtering here as well. That said, stick these speakers in a room where the woofers and walls/boundaries are spaced at distances which are multiples of each other, as well as the listening distance, and you can easily have holes and peaks that will pronounce the problems!
Basically, without getting to into it, the easy solution, besides tinkering with every possible "short wall set up possibility",is to put em in a larger space or set up on the Long wall. You'll likely have an easier time getting fuller more balanced bass if your seating possition is good.
I personally have yet to hear a set up for the IV's/V's where they sounded good too close together, or in smaller more confinded locations (such as along the "short wall".
If none of this is an option for you, your focus/target should be to do whatever you can to ensure all the woofers and boundaries are not at "multiples" of each other.
If I were you, I'd try the long wall set up as a comparison, and also seating location. At least then you can figure what the speakers can do in a potentially better set up scenario.
Don't dispare, the speakers have Bass. But also be aware that your set up is likely the problem..infact, I"m sure of it. I've heard those speakers have excellent bass in larger room layouts than yours. Infact, my buddy has the same problem in his "short wall/smaller room setting" as you with this SCIV's! His bas has sucked sinced day one with these. I told him it's his set up also...he's just too lazy to fix it.