The Van-Alstine mod is at least still marketed, although I guess there's no way to know whether or not they're still provisioning customers. Here is the link to the mod:

http://www.avahifi.com/root/equipment/amplifier/super70i_rebuild.htm

They claim to keep the good stuff and pretty much completely redo the guts, using different tubes even. The cost is fairly reasonable and it has several good reviews.