It really depends on the implementation... In my experience, the 845-based Cary 805C is better than my SixPac with 6 EL34s. Both of these amps produce 50 wpc, and they are designed to operate in the linear range instead of being pushed hard to produce high wattage. The Cary 805 is too rich for my pocket book so I am "settling" for the SixPac monoblocs. I am sure your Mastersound 845 will give the Mystere a run for the money. I've auditioned other KT88, KT90, 6550 based amps (CJ, ARC, etc...) in my system, but the SixPacs won out due to the robust design, leveraging 6 (count 'em) EL34 in triode mode to produce a measly 50 watts. Again, this is just my preference toward the sweet sound produced by the simple EL34 in triode mode. Others may prefer the transparency and clarity of the KT88 or KT90s. Your mileage may vary...
For the benefit of other readers, we're only talking about these tube amplifiers driving the panels and not full-range. The OP's Ethos have internal bass amp to driver the woofers and I have a bi-amped system with SixPacs driving the panels, Classe' driving the woofers.