Phono Preamp - Dual Mono with 24 Volt PS

Is a discrete dual mono with higher voltage power supply an advantage when choosing a phono preamp? 
Not necessarily! The Levinson ML6 preamp had two separate chassis's - left mono and right mono. It sold in 1980 for $6K and was considered the best ML preamp. 
I’m sorry but I don’t think I understand? Are you saying it was a dual mono as well?
A well-designed one-box preamp with phono and line stage and internal power supply can be quite excellent! Many examples abound! Audio Research SP14 comes to mind - there is one for sale now on Audiogon!