You'll never get 15W or more from a 12AX7, even if the valve is in the first stage of the amp section, the effective output power doesn't come from it, but from an IC power section...
Absolutely right, Didier, but you first said that the 12AX7 was in the pre-amp section, which is not correct, because it acts as a power tube in this case, amplifying the signal up to 1 watt only. Later, this signal will be amplified by the VR circuit up to the final power.