With an adapter.  I bought a 4-pin to 8-pin adapter from Angela Instruments that will enable me to use a 5Z3 or #80 in place of a 5U4 and 5Y3 respectively.

I have not felt like taking the adapter to the office yet.  That is where the TABOO is in service. I picked up some 5Z3's last weekend.

To me the 5Z3 and 80 are not exotic. They are generally less expensive than a good 5U4G or 5Y3G with the coke bottle shape. The word is mixed on whether using the older tube through an adapter is better. I have not even tried mine yet.

Yeah, I know I am the impatient sort, but anyone know if this Mullard would work in the Taboo?

I am pleased to report the Mullard GZ37 works excellently in the Taboo. Perhaps not quite as imposing looking as a big 5U4 coke-bottle rectifier, though no shrinking violet, just a bit more svelte. But a very sweet sound.

Interestingly, though slimmer in actual overall diameter than the SED Winged C 5U4G I was using, its geometry permits the use of the same UltraSonic Rx-40 that I used on the 5U4G. Note, the SED Winged C is a bottle-shaped 5U4G, for which Herbie's recommends an Rx-50. I have found the 40 works on the shoulder of the 5U4G well, and around the waist of the GZ37 nicely.