Finally found a back-up pair of 5998s, or so I thought.
I spotted 5998 tubes listed on a website (of a well-known Head-Fi member, unnamed for now) for $100 each. OK, that's a fair price, so I ordered two via Paypal and received order confirmation. The following day, my money was refunded with no explanation besides..."out of stock".
I immediately go back to the well-known website and see the same 5998s listed for sale...at $175 each.
Not cool. The search continues.