Sensitivity 86 v 90

12AU7 is a low-gain tube.  A 12AX7 will definitely provide more gain in a given configuration.  

Most listening takes place around 1 to 2 watts depending on speaker efficiency, distance from the speakers to the listening position, room size and a few other variables.  Musical transients can exceed that..  If your preamp / phono stage gain is low, it will give a false "reading" of the amount of amp power you're using to just get to your typical 1 to 2 watts output, but shouldn't limit transients..  

" but (as I understand it) the gain from the phono preamp can be increased by switching the driver 12AU7 tubes for 12AX7’s, if necessary."

Did the manufacturer of that preamp tell you that? If not you should find out if you can swap to a higher gain tube!

phono preamp is a Rogers PA-2.  Cartridge is Lyra Kleos, .5 mv output version. I agree with the suggestions that the phono output is probably too low.   I can increase it by changing the driver tubes from 12au7 to 12av7 or 12ax7; I will try that. 

@lloydc 

While a higher gain 12Axx tube can drop into the same socket without damage to the tube or the circuit, that's not the same as saying it will sound right. The various 12Axx tubes have different operating points; if the circuit is to bring out the best in them it will be optimized for a particular type.

Ok my recent experience driving 83 db speakers.  Repaired a class A single ended triode amp 28wpc.  To our disbelief, it drop the crap out of those speakers.  So there is much theory about this subject, some times it doesn't always add up.  Go figure. Happy listening.