The Dynaco mod you want is the driver board replacement from VTA. Do the mod they offer and you'll be more than happy. Much more. And you'll be able to do it for a small amount of money. Unless of course you WANT to drop thousand$ on something new and shiny.
Two updated driver boards and parts are around $100. If you don't want to do the work, they can do it for you for $50 labor. It's THAT easy. They also have optional mil-spec caps if you want them, new speaker posts, etc. An upgraded pair of Mark III amps would actually please you better than a Mac 275.
http://www.tubes4hifi.com/MK3.htm
Two updated driver boards and parts are around $100. If you don't want to do the work, they can do it for you for $50 labor. It's THAT easy. They also have optional mil-spec caps if you want them, new speaker posts, etc. An upgraded pair of Mark III amps would actually please you better than a Mac 275.
http://www.tubes4hifi.com/MK3.htm