Thanks! There have been good answers and great answers to queries. Yours exceeds both. Appreciated.
The trick would be finding an upgrade to the WA3 that makes use of my small collection of 5998s.
The 5998, along with the 6AS7G and 6080, are commonly used power tubes so there are other amps out there. The GOTL is one of them, but in its case you need to use a pair of them at a time. This is an even better way to utilize your 5998 collection . You can also have someone other than Glenn build you an amp that uses these tubes. Here is an example of a builder on Head-Fi who can do this:
https://www.head-fi.org/threads/1101-audio-amplifiers-power-conditioners-and-accessories.946679/
I have no affiliation with @A2029 who builds the amps, but they seem to be highly regarded by people who have tried other amps. I'm sure others have recommendations for amps that can run these tubes.