Why on earth do you need a 300W rating? One of those new
Greenbacks are rated at 25W, and can blow quite easy if you exceed that by much. Properly wired, a 4x12 would handle 100W.
If you like low end, you might consider the G12H, rated at 30W each (120 total).
If you like mids and high gain, you might prefer the Vintage 30, rated at 60W each, 240 total.
A popular choice is 2 Vintage 30 and 2 G12H. However, you CANNOT just add the wattages in this scenario. Your total safe range will be 120W, the same as using all G12H.
BTW, Fender seems to be using a lot of Eminence drivers lately. To properly ID the drivers in your cab, you can look at the code on the speaker, then check here:
http://www.webervst.com/codes.html or