Another Mojave alternative is blankenship:
http://www.blankenshipamps.com/data/mainframe.htm
Check-out the Variplex 100 - lots of guys on The Gear Page talking
about these. Replicates the plexi sound, with an amp safe attenuator.
Also, Trace at Voodoo amps makes a Plexi knock-off, suitably named
the V-Plex:
http://www.voodooamps.com/vplex.html
And the list goes on:
Germino Amps - plexis
Metro Amps - variation on a '68 12xxx series plexi
If you are looking for a master volume amp, either roccaforte or splawn
will get you the Marshall sound you're looking for.
http://www.splawnguitars.com
http://www.roccaforteamps.com
I find the Mojave products significantly overpriced compared to voodoo,
splawn, and roccaforte. There's an amp shoot-out on Harmony-Central,
where a guy compared a couple marshalls with the splawn, and the splawn was voted higher than the marshalls (blind test).
Good luck!