After some trial positioning of the spare half of the original IB manifold, behind the Right Main speaker, I took the electric jigsaw to the box again. This reduced it's size to the minimum of 42cm x 51cm x 85cm. Used in the 'tall' vertical format this will house the four 15" drivers safely with some meat still left around the driver holes. The OB baffle 'box' ended up as an open U-shape of still very heavy, laminated 36mm Baltic birch plywood throughout. Having no top allowed easy access for fitting the drivers and wiring them in series-parallel for 8 Ohms.
For testing I just placed a piece of ply on top of the box to close it off and pushed the open back against the sloping ceiling. This left two, large triangular vents to allow very free air flow at the rear. I re-fitted one length of the 10mm bracing studding with large load spreading washers to stop the U-channel from flexing to driver reaction loads. There was no need to seal the bottom as it rests on the carpeted floor of the stage.
Wiring the four x 15" OB to the EP2500 with the channel 2 control at 12 o'clock was completely underwhelming. I had to increase the setting on the Behringer's control knob considerably to make the output suitably audible above the IB. I will have to pull the cable off again to double check that I have all the drivers moving outwards with a 9V battery. I remembered to do it on the IB but completely forgot about checking the OB. [It proved to be correctly wired as expected.] On Netflix films there was a very subtle increase in bass impact. Muting the OB proved that the IB was still doing most of the heavy bass lifting.
Further increasing the EP2500 control to '2pm' and then '3pm' brought the OB bass up for an organ music trial. Suddenly I could hear very natural organ bass. I muted the IB repeatedly to better hear what the OB was contributing. This was more like it! The big pipes sounded suitably "breathy" and very deep. Still no BFD filters at all. This would require I fire up REW to get some idea of the raw frequency response. From long experience with them working in an IB I know the older AEIB15s don't do serious infrasonic bass. At least, not without heavy boost. That doesn't mean they can't easily reach 30Hz with plenty of "welly" to help the IB along and hopefully provide a bit more impact on drums and bass.
Playing a variety of music I gradually brought down the levels of both OB and IB to avoid a rather detached, "heavy-footed" effect. While Netflix benefited from rather more bass for more impact. The OB box lid was given a seating of strips of foam to kill vibration and to provide a better seal. I shall provide a second layer to bring the lid up to 36mm as soon as I source some more Baltic plywood. It is important to trap one layer of 18mm plywood between the sides to reduce flexure in the manifold sides. REW testing proved a central shelf to be well worthwhile in a manifold even when a solid 36mm top and bottom were already in place.
The only way to get more impact is probably going to need a higher roll off point on the the OB. Since the Onkyo TX-NR818 cheats and connects both subwoofer sockets internally nothing can be done about a separate roll off point for only one sub. So it will require raising the roll off point on the Main speakers to push [both] subwoofers higher in frequency. With the faster roll-off of the newer drivers in the IB this will not affect it much. The question is whether I really want a higher cut off point on the 40Hz[-3dB] floor-standing, Mission 753F Main speakers. Having revisited the speaker setting menus on the Onkyo it seems there is very little individual adjustment available. LFE cut-off can only be set as high as 120Hz. This will have no effect on "ordinary" bass signals. Only on LFE.
Raising the "crossover" frequencies of the speakers just made the sound rather thin. So I reset the Mains and Centre to 80Hz with surrounds still at 120hz. Listening to Celtic Circle 2 I noticed a considerable increase in impact on the large drum on one track. It took me quite by surprise after probably 100s of hearings.
Overall I think this four x 15" [manifold] OB more worthwhile than any previous attempt to use the older, plastic coned drivers. I am still muddling over how the OB might be allowed to play much higher frequencies as well. I can't use the right Main signal because that is rolled of below 80hz. Subwoofer-out from the Onkyo Rx can't be independently adjusted and [I assume] will depend entirely on the crossover setting of the Main speakers.
Still having made no further adjustments I am noticing slightly greater impact from the OB on bass drums and bass effects. The effect is usually quite subtle.
Click on any image for an enlargement.