I don't have any experience with the JJ or EH tubes to give any guidance. I think as far as small signal tubes go, the JJ and EH they are good tubes. I do know however that in certainly applications, (receiver phase inverters for example), the JJ tubes don't like the stock design and perform poorly -- poorly as in clipping at slightly above normal volume levels. In the receivers however, Fisher switched to a 12AX7 that is operated nearly into the cut-off region. The JJ tubes apparently don't do well as that region is approached in equipment designed for the original vintage product. In the X-202 however, the design is much more traditional, so that any tube attempting to emulate the original 7247 specification should do fine.
Using a 12AX7 in place of the specified 7247 won't change the relative volume level of the unit's operation at all, as the section of the tube that operates as the inverter section inherently operates at a unity gain, regardless of what tube type is installed.
Dave