For the OP's benefit (in case he or she doesn't know), 92 dB sensitivity generally means that one such loudspeaker will deliver a sound pressure level (SPL) of 92 dB at a distance of 1 meter away from the speaker when it is driven with an input voltage of 2.83 AC volts. 2.83 VAC is 1 watt into an 8 ohm load -- and the
nominal impedane of the Mission 707 is apparently 8 ohms.
That said, the
actual frequency dependent
impedance curve of the speaker is also going to be important into guessing how well, or poorly, it is likely to mate with a vacuum tube amplifier. Why? Mostly because
most vacuum tube amplifiers (for several reasons) have relatively high output impedance (i.e., relatively
low "damping factor"), which means that the amplifier tends to "feel" the load more than would an amp with high DF. This
can have audible consequences if the speaker is one with a
complicated impedance curve.
https://www.hifiengine.com/manual_library/mission/707.shtml
http://www.hifi-review.com/153684-mission-707.html
The sensitivity of the Mission 707 was even higher than rated. An input of 2.83 volts of pink noise in an octave band centered at 1,000 Hz produced a 94-dB SPL at a 1-meter distance. The system's impedance curve had maxima of 25 to 35 ohms at 23, 95, and 1,500 Hz, minima of 7, 6, and 5 ohms at 55, 200, and 8,000 Hz, respectively. Given this performance, the impedance probably should have been rated as 6 ohms, but the manufacturer's 8-ohm figure is not unreasonable.
That "6 ohm" figure probably explains, at least partly, the higher measured sensitivity in the test above. 2.83 VAC into a 6 ohm load is 1.33 watts, which could amount to 1.25 dB
more SPL, all else being equal, from a drive of 2.83 V.
These are the kind of considerations responsible for, I think,
much of the vaunted
synergy between at least some hifi components (amps and speakers, in this case).
Practically speaking, what kind of vacuum tube amplifier is likely to be a good choice for these speakers? Well... hard to say.
A push-pull EL84 amplifier can provide somewhere between 10 and 15 watts (per channel). All else being equal, this would correspond to an SPL of up to about 103.8 dB (for an input of 15 watts) at a distance of 1 meter. That is
loud. Note that this information says
nothing about the quality of the sound, just the volume
I
am going somewhere with this
I would suggest that a push-pull EL84 amplifier would be a good place for the OP to begin his/her consideration of vacuum tube amplifiers, if an amp is the desired component. Best of all, EL84 amplifiers, IMO/IME, sound
good. It is hard to go wrong with a push-pull EL84 amplifier (even with "modest" output transformers), again IMO/IME.
Something like
this (which is an "integrated" amp in the most nominal sense of the word), e.g., might be worthy of consideration. The JoLida brand's been around for a
long time (although there was some ugliness some years back between the US-based JoLida and a Chinese company using the same name). The JoLida hardware
is Chinese made, but there is (still, AFAIK) a real (
albeit small) US company standing behind the products.
Here are a couple of products at a couple of price points, e.g.
https://www.musicdirect.com/integrated-amp/jolida-fx10-integrated-tube-amp
https://www.musicdirect.com/integrated-amp/Jolida-Fusion-1102-Integrated-Amplifier
That latter amp is said to product 25 wpc from push-pull EL84 -- that is not
impossible, but requires running a pair of EL84s pretty hard. There are "uprated" EL84 tubes (vintage type in the US known as 7189, more modern Russian equivalents known as EL84M or 6p14p-ev, Cyrillic 6П14ПEВ) that might be a better choice in such an amp than plain vanilla (modern production) Russian or Chinese EL84s.
https://www.tubedepot.com/products/sovtek-el84m-power-vacuum-tube
(one 'modern' example, e.g.)
Of course, the canonical choice in "medium-power" vacuum tube amplifiers would use a push-pull pair of 6L6 tubes or their descendents, e.g., EL34 or the "kinkless tetrodes" (KT-xx)... but such amplifiers are considerably more expensive and, to me, with few exceptions, not as
soulful sounding as many EL84 or 6V6 amplifiers.
Hope these ramblings are helpful, or at least interesting