I think one point everyone is missing, is that a good quality speaker shouldn't
need a bunch of parts to "fix" the response of the drivers. The Harbeths did measure quite well, and are known for their warmer tonal characteristics, which may not be for everyone.
And Danny tests them as they are given to us by a customer. We often find that the response is better with the grill off, tho the Harbeth is an exception to that general rule.
With some speakers, there's nothing we can do, or what we can do will cost $700-1000 to "fix" and even then, its not a guarantee.. Especially if there are phase issues that can't easily be addressed.
Sometimes, the tweeter has limited extension down low, and the woofer cant play up high enough to meet it without also introducing break-up or ringing in those ranges.
Othertimes, it may be a good candidate, but it's size is an issue, and who wants an outboard crossover that they now need to find space for?
We currently have 2 different versions of the same speaker from the same manufacturer. but the $3000/pair versions measure like garbage while the $2000/pair versions measure pretty well.
Decent quality parts in both of them, one of them clearly needs a lot of work.
Our upgrades are very dependent on the speaker, and also the response to our feedback from the customer, and there are several upgrades Danny has done in the past that he doesn't offer an upgrade kit for on the site.
We've had some speakers brought to us that may have been good candidates, but the customer decided against upgrading them after listening to some of our kits.
Here was the crossover of another speaker we didn't upgrade:

It had the same space issues as the Harbeth, even though it was a bigger cabinet, but it was full of compartments full of thin bracing and foam.
And sometimes we likely shouldn't upgrade a speaker, but the customer wants us too for nostalgic reasons.