Well, I went ahead and ordered some solid-core speaker cable from Origin Live. Only £6.60 per metre and you can send it back for a refund if you don't like it.
This stuff is, frankly, a complete b*stard to fit. It's as thick as hosepipe, heavy as lead, and about as flexible as a frozen snake. Fitting the bare ends onto the teeny tiny terminals on the back of my speakers (2 out of 10, Mr Klipsch - see me) was a nightmare. It took the best part of an hour and a lot of swearing to get the damn things connected.
It was, however, worth it - Messrs Zanash and Colin2040, please take a bow. Solid-core is indeed the way to go. Compared to my old cable, I'm now getting more clarity and better soundstaging. It's not a huge 'night and day my god I've had a religious experience' kind of difference but it's certainly there and it's all good. Best of all, though, the BOOM has gone. Bass now sounds full and controlled and defined with none of that 78 stone wobble I was getting.
My best (admittedly ignorant) guess is that the old Chord Flatline stuff was simply not well balanced, with a decided bias towards the bass. Anyway, I am now a very happy bunny :JPS:.
BTW, re comments on speaker/room interaction causing boom, I heard this the other night on a mates setup. He'd moved his speakers to accomodate new furniture and turned a lovely sound into a big boomy disjointed mess. Fiddling with different cables did nothing at all to alter this, but fiddling with speaker placement and stuffing the ports with socks sorted it out a treat.
So, thank you to everyone who posted, and in particular to those who pointed me at solid-core cable and speaker placement. It's another winner from the gentlemen of ZG