I read your post several times, I am confused if you are actually looking to upgrade the pre-amp, or if you should be trying to upgrade your amplifier?
you love the Shindo, so try a different amp.
Try a Coda 16- my 15.5+ sounds exceptional with my 3.6r’s. 100 watts per channel of class A, then it transitions into class AB 150 wpc, 100 amps of current your maggies will appreciate, and it doesn’t get hot. Have not heard their 07x preamp, but have read that it is warm as tubes, and obviously matches well with their amplifiers.
The coda 16 has 50k unbalanced input impedance, but as suggested above maybe you are in the market for a dedicated phone stage?
that may satisfy your "curiousity".