I’ve found cabs that use a properly crossed-over mid driver in leu of or in addition to a tweeter are the smoothest I’ve encountered.
Examples of those that I’ve used are a pair of Baer 12/6 cabs, an older Dr.Bass 212/6/1 and a very old EV B15M. All have very smooth mids and highs and a smooth transition from woofer to mid driver. In my case, I modified the EV cab to use modern drivers and improved the crossover, extending the cabinet’s range and power handling and making the transition even smoother.
Other cabs I have tried that exhibit these characteristics are AudioKinesis, fEarful/fEarless Greenboy designs and, to a lesser degree (not as smooth or natural but not bad), the Avatar 12/6 but I’ve only tried these cabs out and haven’t clocked enough time on them to really judge them like I have the above cabs.
Bergantino makes great sounding cabs with great clarity but I don’t think they’re as smooth as cabs with dedicated mid drivers (just my take). I also find them to be a bit scooped.
Epifani cabs lean towards more highs and a less scooped sound than Bergs but I, personally, find them less smooth.