There were two problems in the old days. First, the sockets were junk and only had about 5000-7000 hour service life, then they had to be replaced. Any amp that uses the ceramic sockets has this problem. The new amp uses Teflon sockets and holds up fine.
The second was we were concerned about continuing availability of the tube itself. This is still an issue, but we are only doing this as a limited run, so no worries- there are tons of these tubes around!
Older amps can be updated for about $3700, with the warranty reactivated.