I want the tweeter upgrade because I like a lot of zip. I do a bunch of slap bass tunes and a pronounced tweeter is really nice to have. I currently run the tweeter on max and still dial in a lot of treble on my amp to get the sound I want. It gets there but Nick at Xsonics said I can expect a 3 db increase with the upgraded Eminence tweeter. That should bring this cab closer to fitting my needs. I'd say for most people, the stock tweeter would be fine but if you're a "tweeter" kinda person, you may want to opt for the upgrade.
I was wondering how these cabs would work for folks who needed to do more slap and get more crisp highs out of their rigs. I find the tweeter very nonpronounced in my cab, which works great for me with classic rock, blues, modern country and some folk. With the tweet upgrade and the already solid sound of these it should work out great for you, and I look forward to your opinion once installed.

