One to look into would be a Eminence L'il Texas. 100 watts, 4 lbs.
Jensen has some new Neodymiums, too. I like the 65 watt for a clean Fender-type sound, with some moderate built-in breakup (believe it or not, EVs have that, too), and they make a 100 watt model as well. All 4 pounds.
Although you didn't specify which mark series, Google this:
"mesa mark iii manual".
Scroll down to Maintenance and Service Guide.
This explains which tube does what.
Enjoy.
Also, this:
http://www.thetubestore.com/Resources/Guitar-Amp-Info/Gain-Factor
This explains tube differences, like 12AX7 vs...
FWIW, none of my original factory EVM 12Ls have gaskets for front mounting.
Good to know about the Neo, have to try 'em in a Thiele or two.
I have two L'il Texas (four lbs!!!) in a Twin, they're good, but neos tend to sound a bit bright to me.