I had originally designed the circuit with 7HC595N Shift Registers (my favorite "default" shift register) without considering the large current required for the Numitron tubes. I had intended to either multiplex them or have all 6 tubes displaying at once, but even a single tube displaying a single number would be ~100 mA, which was waaaaaay more than the poor little 7HC595N shift registers could put out. Not to mention that the ATMEGA chip can only sink about 200 mA max on each ground line. I wouldn't have noticed this problem at all and probably fried at least one of my shift registers had I not noticed that each number displayed on the Numitron in a test circuit was a slightly different brightness. A number such as 1 would be super bright, since it was only 2 segments lit, whereas 8 would be super dim, since all segments were lit. In frustration, I began my search anew, after having already purchased 10 of the 74HC595s to use in the circuit.

Someone online at one of the Arduino forums had figured out that a TPIC6B595N shift register with high current capabilities could easily sink enough current to handle a Numitron tube. The only problem was that any shipper in the US charged about $5 per chip with quick shipping, and any shipper in China had slow shipping, but ~$0.50 per chip. I went with the Chinese seller, and was pleasantly surprised with the quick shipping. Now I just need to redesign the circuit so that I can use the 74HC595s for the Nixies and the TPIC6B595Ns for the Numitrons.