It is my understanding that the silver Earmax is technically identical to the anniversary edition, with the exception of the NOS tubes.
Cosmetically, there are huge differences as pointed out by RobN and others.
These differences, along with the apparently non-limited edition of the silver, explain the price difference.
The Tube Amp Doctor is an awesome shop. I have known these guys for many years, and I hang out with them whenever I can, in fact very recently in December 05.
They are the only ones that are allowed to work on my guitar amps, because they do a great job every time!
Their tubes are awesome; for the money it will be hard to find better (matched) tubes than those that the Tube Amp Doctor specifies as "STR" (=Special Tube Request).
Read their descriptions of these STRs carefully; they really try to describe characteristics and heritage of their designs. STRs come as preamp and poweramp tubes.