If you have a mismatch, wouldn't it be easy to just install a resistor in parallel across the speaker terminals to lower the impedance that your amplifier is seeing? Or use the optimal amplifier - an OTL.
Conventional tube amplification has a transformer to reduce the native output impedance of amplifier to the 8 ohm standard. OTL means output transformerless. In order to avoid output transformers and the problems they can introduce, some people choose to use an OTL and appropriately high impedance speakers. Since you have the 12 ohm speakers, you should talk with Atmasphere. He makes the amps you need.
Conventional tube amplification has a transformer to reduce the native output impedance of amplifier to the 8 ohm standard. OTL means output transformerless. In order to avoid output transformers and the problems they can introduce, some people choose to use an OTL and appropriately high impedance speakers. Since you have the 12 ohm speakers, you should talk with Atmasphere. He makes the amps you need.