Going on nothing more than the frequency response graphs, the 1996 model looks like it's a rather laid-back speaker, esp with everything above 15K rolling off pretty quickly, creative a somewhat warm & inviting signature.
Meanwhile the 2006 speaker looks to be a fair bit hotter on the top end, above 5K & especially so above 10K, which may be leading to the more clinical or lifeless tone.
Something else to consider may also be system synergy, where say sometimes amps that sound great with one speaker, may sound cold, brittle or harsh with another, or could swing the opposite direction leaving the speaker sounding muddy or dull or overly warm.
But it's hard to say for sure without A/B swapping different parts of the chain to see how it is affected.
Heck it could even be down to driver materials, where even measuring similar, may have a difference in texture or tone that can't be accounted for with just a frequency response.
There's a lot of variables that can be difficult to account for.