Help with increasing gain on pre amp or options?

Hi,

I've just recently paired an AR LS15 tube preamp with a pair of Cary V12 Monoblocks and need to increase the gain from the preamp to get more volume....probably around 30% including some headroom.
What is the best was to do this?
The LS15 has the remote volume control so I'm guessing it's not as simple as changing a couple of resistors?
The issue seems to be with the V12's, since I had the same problem with lack of volume when driving them directly from a Monarchy DAC, and I've driven other amps directly from the DAC without issue (both LS15 and V12 are new to me, so I haven't had experience with these issues before now).

Is it possible to change output tubes in the LS or input tubes in the V12 to increase gain?

I don't want to open up the V12's if possible.

Any help would be appreciated.

Thanks

Rooze