I had a Musical Fidelity B1 from very early in its production run. I bought it because it was reviewed in the hardware review section of The Gramophone, an august mag that was 80% reviews of recordings [of which +/- 70% 'classical'] and 20% reviews of hardware commensurate with the quality of the music.
The comment that summed up the review was that this amplifier was "unnecessarily good". And it was very good. I had it for 25 years.
I saw no reason to 'upgrade' - ever. For the class of speaker I had - Rogers, Mordant-Short, Monitor Audio, it was, along with that other all-time great, the Marantz CD63 Mk 2 K.I. a wonderful reproducer of music. I got so worried that it would fail permanently that I 15 years later I bought a spare. I never needed that.
I'd still have it today if it had the digital connections I now need for my present set-up, with optical out from the TV. and BuRay.
I don't know because I never A/B'd them as you have A/B'd the Cyrus pair but I bet it would have kept its end up or perhaps surpassed the 'default' Marantz 6006 series that is its successor.
I see that the original Mission Cyrus 2 can be had for about £350 s/h. As the latest amp doesn't have digital connections, owners of the original won't be buying the new one, if they need digital. Would the additional £650 needed to buy the current amp be justified in terms of the quality of sound? I doubt it.