Trial and user error!

Two hours wasted on REW later and I was getting nowhere. I must have run 100 sweeps and still had no improvement. Thankfully, the limited number of sweeps before one has to delete earlier sweeps has gone. Thank you for that priceless improvement to your free software.

At one point I had a perfectly straight and level response from below 30 to over 110Hz. Then I made one small adjustment on the BFD and never got that flat curve back. So I ended the session without having achieved anything useful.

The following morning and I decided to reset the BFD to factory settings for the first time. I was getting odd effects which seemed intractable to any change. The reset produced a number of flashing red lights and a complete cancellation of all preset filters.

I soon found that I had been changing one IB set while imagining I was filtering the other. Quite inexplicable but true. I resolved to write down the channels affected by each box in the equipment chain. I had reversed the speaker cables from the EP2500 power amp since changing the position of the driver sets.

Behringer mixer, CX2310 and BFD (from top to bottom) 
It is usually too dark to capture the rack components without flash. 
So most of the usual lights are invisible in my pictures.

Behringer do not make it any easier by calling a channel one or right at random on their equipment. Of course the way the boxes are wired together will change which channel is right or one or two or left. It turned out the BFD was now reversed compared to all the others. One set of four 15" drivers is on each channel. The BFD and mixer affect only the bass channels. The speaker channels do not pass through either.

With two channels to play with REW is best run with stereo (or dual mono) connections. I use RCA/phono to stereo mini-jack, solid adaptors on the twin core, cable connections. This ensures that both sets of drivers can be tested together. Or one set alone, simply by muting one set, or not, on the CX2310 active crossover.

This is the result of another two hours of REW sweeping:

The bass is around 10dB hot but can be easily reduced on the active crossover gain controls to taste. Black line is the full combined IB with main speakers. Blue line is the four newer drivers. Violet the older four.

It looks quite a mess but will be further refined later. I now prefer to have a gentle hump between 30 and 60Hz to get some power and impact. My decades of chasing the infrasonics at the expense of higher bass frequencies are over. The IB has more than enough infrasonic power to wreck the house without worrying about silly house curves.

The peculiar, signature hump at 13Hz, with the associated higher frequency trough, is a known room effect. The older IB15 drivers are still getting their customary 20Hz, +16dB boost with 120 B/W. The newer drivers about 3dB boost at 20Hz. I also gave them a gentle lift in a daisy chain of gentle boost filters from 30-60Hz. Without which they follow a steep downward slope with rising frequency.

I tried every imaginable crossover point but nothing will affect the trough at 170Hz. This is obviously another fixed aspect of my system layout relative to the room. Previous testing shows it to be a cancellation effect between the IB in the left wall and the right, main speaker.

The sound quality is back to normal as I listen to my favourite organ CDs at fairly low levels. The clarity of the bass, as it ponders up and down in the acoustic basement, is exquisite. Even at the threshold of audibility the timbrel character of the great pipes is effortlessly portrayed.

Click on any image for an enlargement.