"Wally" wrote in message
...

Rather than the idea I mooted recently about making compact transmission
lines using Kef B139s and driving them with a filtered signal to provide
lift where the speakers roll off, I have another option - I have a total
of
four B139s, two in the existing speakers, and two bought as spares and/or
drivers for the future bass units.

Am I right in thinking that some sort of push-pull set up will give me a
shallower roll-off for a given size? What design approaches are there, and
how do they compare?

The simple fact is that by modern standards, B139s are pretty mediocre
subwoofer drivers. They were never much more than fairly good plain old
woofers.

*real* subwoofer drivers don't need kludges like "isobaric" cabinets to
outperform any reasonable requirements.

If money is tight, auction off the B139s and use the cash to buy a real
subwoofer driver.

Oh, and if you are building a subwoofer, forget transmission lines. They are
basically a waste of good box volume.