Supermagoo,
You can do same simple circuit as you do with a single NPN transistor that have a inverted input, but maybee 10 times up or more, with the input serial mix resistors and the feedback resistor.
(use the second 12AX7 tube half as a dc-coupled cathode follower after the first inverted gain amp, and you get a better output drive)
Can maybee work with decent result up to 8 mixed input resistors, each with same value as the feedback resistor.
(because the low open loop gain in the bus/mix amp, less than 30 dB for a single 12AX7 tube half)
If you want a better sum/mix amp for more inputs, you can build a differential amp (op-amp) with two dual triodes, (four tube half) so you get more open loop gain.
Sorry, I can not write a on-line schematic just now, but maybee some one have a application scanned, for this I have talk about here above ??
--Bo