In my experience for most speakers, the relationship between sensitivity and amplifier watts is often an overblown consideration. Gain, however, is predictable. Like others have stated you may have a gain issue. I do not know your preamp but 4-6 o'clock seems quite high already and a lower sensitivity is sure to make that more challenging.
Sensitivity 86 v 90
I am considering an upgrade from speakers with a sensitivity rating of 90 db, 4 ohms, to larger speakers rated at 86db, 8 ohms nominal. Same brand, PMC. My tube integrated amp is 80-112 watts triode/ultralinear, and it’s fine for my 90 db speakers. Although, it is sometimes at around 4 o’clock on the volume control, approaching the max at 6. I am aware of the “amp power must double for each 3db increase in volume” rule of thumb, but really have no practical experience with this. I do like having 90 db efficiency, always assumed that meant a less powerful amp would suffice.
My question is, would the decreased efficiency be a concern?