Need Help With Input Sensitivity/Watts

My preamp puts out 0.7v into a 3.5 watt SET with an input sensitivity of 500mV. With my speakers I can drive everything really loud without any sound degradation so that's not the issue. My question is this: I'm looking at another power amp which is rated at 8 watts but with an input sensitivity of 800mV... Will the outcome be similar? How does the whole sensitivity/watts equation work? Currently (see what I did there...) I have the preamp's volume at 11/12 o'clock for ideal listening levels. What will happen with the 800mV/8 watt amp? Thanks for your help!
Thanks Al, that explained it perfectly. Normally my eyes glaze over when I try to make sense of system math.
FWIW a preamp that can only deliver 0.7 Volts is not much of a preamp!

Most preamps will make at least 2 volts (and many make much more than that), so in general if your amps are as sensitive as claimed (FWIW most SETs are less sensitive than that) you should have no worries.
The results will be very similar in terms of volume control settings, assuming the specs are accurate, and assuming that the impedance of your speakers is not a great deal lower than the load impedance the amplifier power ratings are based on.

The ratio of two power levels converts to db as 10 x logarithm(P1/P2), for a given impedance.

The ratio of two voltage levels converts to db as 20 x logarithm(V1/V2), for a given impedance.

8 watts is 10log(8/3.5) = 3.6 db greater than 3.5 watts.

800 mv is 20log(800/500) = 4.1 db greater than 500 mv.

For a given volume control setting the difference between the sound pressure level (SPL) produced by the new amp and the SPL produced by the existing amp would be (3.6 - 4.1) = -0.5 db, which is a negligible difference.

This neglects the possible effects of differences in amplifier output impedance, which will also be negligible unless the impedance of your speakers is much lower than the load impedance the amplifier power ratings are based on.

The 0.7 volt output spec of the preamp is not meaningful unless the input voltage to the preamp that is required to produce that output voltage is also specified, and the output voltage of the source component is known, but given the volume control settings you are presently using it is undoubtedly not an issue.

-- Al