OK - small scratch in the veneer on a really prominent part of your 'Logan - "slight" defect, but very significant all the same!!
The damage is "small in degree", but "sufficiently great to be worthy of attention" - especially to those of us that treat our speakers better than our wives!
If you were looking at purchasing those Logans and were negotiating a price, would you consider that scratch to be a slight defect? I don't think so. But this example is definitely better than your last one. I'm beginning to see your point. You are saying that an improvement in sound can be both small in degree (slightly tighter bass), yet sufficiently great to be worthy of attention (the bass sounds significantly better to me with the added tightness).
Ultimately, I still think "slight but significant" is a horrible choice of words when describing a perceived improvement in sound. Either it was a slight improvement or it was a significant improvement. Or maybe it was slightly significant.