1978:
Try to hook up any speakers from 700 £ up ( for example ) to a HI-FI costing 100 £ and play any CD - then swap the speakers for the ones which come with the stereo..then hook up these cheap speakers to a hi-fi costing 700 £...you'll hear the difference.
Are we talking mini-system type stuff here? And the £100 hi-fi, is that £100 all-in or per item? Or are we talking used kit that costs £100...or brand new, or just a perspective that fits your view?
I mean, if we're talking about the kind of systems that use the equivalent of old cereal packets for the their speakers, I'm not quite so sure I'd like to hear what the rest of the thing sounds like anyway and your argument still doesn't stack up because it's going to be rubbish in, rubbish out. The great speakers won't make the crud sound good. It's still going to sound like crud. Been there, tried it, it doesn't work!
It's all about balance between the three and not subjective references - I've swapped my kit around wholesale this past couple of years and each change made an equal impact. Amp, CD and speakers. The biggest change, arguably, was by moving the speakers around.
Incidentally, the sound quality you get from a £100 DVD player isn't a million miles off a good budget CDP - the transports are fine, the DACs are mass produced and are often found on more expensive players, so it's an easy position to adopt that putting a cheap hifi onto pricey speakers will sound good, but great hifi on rubbish speakers won't. Misses the point entirely I think. It's way harder to make great speakers for little outlay, barring a few models that spring to mind (Roth, Q Acoustics), as design compromises on the speaker will be more readily apparent.