from Sphinx to Cronus Magnum?

Why not upgrade to a Brio R? If you look at the million page LS50 thread on AudioKarma it appears that is the match made in heaven and completely does justice to the speakers.

Taking off on what Al said there is not much in the way of lab tests on the integrated, but if we take a leap of faith and assume that it is comparable in output stage to the Titan Magnum the test is here http://www.stereophile.com/content/rogue-audio-titan-atlas-magnum-power-amplifier-measurements
and the output impedance measures a very moderate half ohm on average.

On the good side this will mitigate frequency response variations somewhat due to the voltage divider effect. On the other hand it indicates a reliance on feedback and, no doubt, one will either be comfortable with that, or not.

Anecdotally, there are a number of people reporting good results with tube amps on the AK thread, but YMMV. Then again some people listen to Klipshorns with Phase Linear amps so anecdotal reports may not be all that they are cracked up to be.

May I suggest adding a good subwoofer?
A good musical foundation, or "base", gives your ear/brain something to compare the mid/highs to for a more natural perspective.
Really good bang-for-the-buck.

Thanks, all, for your insights here which are helpful! And, Al, I love the pictures of your listening room. Looks amazing!

cheers, Margot

Hi Margot, did you end up upgrading to the Cronus Magnum? Wondering what your thoughts were. I'm also thinking about moving from the Sphinx to the Cronus Magnum.

I've listened to both the Sphinx and CM2 at the dealer and the sound quality difference was very noticeable.  The CM2 to me sounded a lot better in every way - cleaner highs, mids and better bass. I ended up spending the extra money for the CM2. For the money, the CM2 is a solid amp and it also looks great to add...you can't go wrong with it.  There are a lot of great amps out there...take your time and listen to a few.  Good luck!