Not really comparing apples with apples here. Maybe comparing the Mystere which is an EL34 or KT88 amp with the Ref 3.1 (KT88) rather than the 3.1(300B) might be more informative.
I haven't heard the Mystere but have listened to the Ref 3.1 (KT88) and thought it was stunning driving a pair of Living Voice Avatars. Very neutral for a tube amp providing more detail than you might expect and very good integration top to bottom. The bottom end is reasonably tight though doesn't match some ss amps I've heard. My regret was that I'd already bought the Cayin A-88-T which, while also an excellent KT88 amp, was playing in a different league to the Audio Space (at an appropriately lower price to be fair). I live in Hong Kong so it may still be somewhere in my future!
I haven't heard the Mystere but have listened to the Ref 3.1 (KT88) and thought it was stunning driving a pair of Living Voice Avatars. Very neutral for a tube amp providing more detail than you might expect and very good integration top to bottom. The bottom end is reasonably tight though doesn't match some ss amps I've heard. My regret was that I'd already bought the Cayin A-88-T which, while also an excellent KT88 amp, was playing in a different league to the Audio Space (at an appropriately lower price to be fair). I live in Hong Kong so it may still be somewhere in my future!