For reasons of portability, I frequently use a Mesa Boogie Mini-Rectifier and 10-inch speaker, which I can easily bring on a plane or load onto my motorcycle to ride to gigs. The drawback is that this amp-speaker combination often lacks in bass response and head room, which are really crucial elements to my clean sounds.
In real gig situations, I need lots of volume to keep up with loud drummers. You can always increase the treble on most amps, particularly Mesa Boogies. But to my ears, there is nothing uglier and more painful than boosting the sharp treble end of the frequency spectrum just to be heard. The original Mesa Boogie power tubes tended to break up way too early, not allowing me to generate clean sounds with punch and clarity and medium or higher volumes.
On reading the description of the CZ tubes, I decided to give them a try. I'm VERY SATISFIED. While they aren't drastically louder, they are noticeably louder and deliver more bass, allowing me to stay clean at higher volumes without resorting to boosting ear-piercing treble frequencies.
Most of my distortion is derived from pedals and with the CZ tubes, so I can now use them to push the power tubes to distort at a more appropriate volume level for a gig situation. Contrary to some reviews out there that the CZ tubes lack character, they sound warm and smooth. On the other hand, the TAD EL84 STR tubes, which are touted for their tone and claimed as being superior to the CZ tubes, they are no better than the stock Mesa Boogies I had--they break up too early and lack bass response. I will definitely keep a spare set of CZs on hand!