Clive,

can a digital EQ system like the Behringer, working only at low
frequencies (say below about 300Hz) be expected to improve matters

In a word, No. The only viable solution is getting bass traps. That will
reduce the peaks, broaden their bandwidth (which reduces the effect known as
"one-note bass"), raise the nulls, and reduce ringing. And all four of those
will be improved everywhere in the room. Unlike EQ that might reduce peaks
in one place, but make them worse elsewhere.

It's impossible to make any small room perfectly flat, but you can
definitely go from "This is terrible" to "Wow, that's great!" with good bass
traps.

--Ethan