Thanks, Mofi. Yes, I agree that despite the somewhat low input impedance of the amp, it is very unlikely that there would be an impedance incompatibility with the preamp.
To be precise, the rule of thumb guideline of 10:1 or more should be applied to the highest output impedance the preamp may have at any audible frequency. That is especially important in the case of most tube preamps, for which output impedance commonly rises to much greater levels at deep bass frequencies than at the mid-range frequencies for which it is usually specified, due to the coupling capacitor most tube preamps use at their outputs (the impedance of a capacitor is inversely proportional to frequency).
If the preamp has been reviewed in Stereophile, the measurements John Atkinson usually provides in conjunction with the review will indicate the worst case (highest) output impedance at any audible frequency (which is usually at 20 Hz). If that information is unavailable, I suggest applying a factor of 50:1 or even 75:1 to the manufacturer's spec be completely safe. In this case you would be at 110:1, which certainly seems ok.
Not meeting those criteria, btw, does not mean that there will necessarily be an impedance compatibility problem. Meeting those criteria, however, assures that there won't be an impedance compatibility problem. If the criteria are not met, there may or may not be a problem depending on how much the impedance varies as a function of frequency.
Regarding gain, power amp specs very often do not include a specification of gain. However, a good approximation can be calculated from the power and sensitivity specs. See my post here
for how to do that.
In this case the relevant specs for the amp are 150 mv sensitivity for 1 watt output, apparently into 8 ohms. That calculates to a gain of 25.5 db. It also means that about 2.1 volts into the amp will drive it to full power (200 watts into 8 ohms).
All of those numbers, as well as the 12 db gain of the preamp, seem reasonable and suitable for use with 90 db speakers. Although, as is often the case these days, if you are using a digital source having a rated maximum output of 2 volts or more, and your listening is done at relatively low volume levels, you might find yourself operating the volume control in the lower part of its range. But I suspect it is unlikely that you would find yourself operating the control so far down in its range that it would be bothersome.