I had to go through a bunch of speakers before I found a match for the Z28 that suited me. The amp is halfway between a British and an American sound. ( The crunch sound is in the Marshall camp). I ended up with something quite neutral - a ceramic Tone Tubby 12". It's great but expensive. I hated the Z 10's that came with the amp - the huge mid-range hump sounded honky to me. The G12H30 had a nice punchy, clear bottom but the highs were a bit harsh. I thought an American-voiced speaker would bring out that aspect of the amp so I tried a Weber American-style speaker (12F150, I think). That was the worst of the bunch - a terrible match. Used an EVM12L - didn't like it and even if you do like that sound you'll find them to be very efficient so they won't solve your loudness aversion. They say the Greenback works very well with the 28. I haven't tried it but that is the Doc's first choice with this amp.
To be clear, I like all those speakers and have used them all for years with various amps. Also, you should know that I auditioned those speakers with high-end speaker and instrument cables. They make quite a difference (with high-end amps) so YMMV.
FYI, The Celestion Gold, Blue, Weber Blue Dog & Blue Pup and all other variations of the Blue will give you a distinctly British flavor.
If you like the V30 try it with a Z Brake-Lite (low attenuation only). I suggest trying a Greenback. After that a Tone Tubby ceramic.
THE STUFF I'M HAPPY WITH:
James Tyler SEHD
Allen Amplification Accomplice
Vovox, van den Hul and Sommer cables
Area 51 wah
Goodrich volume pedal
Tone Tubby ceramic 12"
mid 60's Gibson J-45
J Design cab