MST3K_ServoLooks like 12AX7 and 6V6 version are the western replacement tube type...
With tube amps, especially ones with output transformers, two things matters the most:
Tubes: Swap in a good set of tubes can mean the difference of night and day. Kinda like the engine in your car.
Transformers: Everything goes through the transformer before it goes to your headphone/speaker... so kinda like your car's transmission.
That said, this is assuming the topology and setup is done correctly. (like the car's chassis).
MST3K_ServoI'm a bit late here, but someone on the Head-Fi forums talked to the manufacturer. They said the amp puts out 12.6V and 0.6A to the pre-amp tube. That means 6V-only tubes like the 6DJ8/6922 are not compatible. 12V-only tubes like the 12FQ7 and dual voltage tubes like the 12AX7 family and 12BH7 are compatible.
Alfa24VIf one wanted to use a 6v filament tube, it would be easy enough to pad down the filament voltage with an appropriate resistor. I recommend trying the E80CC or E180CC, both of which are excellent Philips 'special quality' tubes and fall more in line with the 12A_7 family operating parameters than the 6DJ8 types.