Well, since Mr. Marcus Daniel of Mesa support was so kind and shed light on the matter (while nobody here knew the solution, or didn't care, for what's worth) I thought I should share the knowledge with the community. My initial inquiry was:
"Hi there guys,
I am the proud owner of multiple Mesa items and my newest addition is a Rectifier Pre. Now, I'd like you to tell me more about the modifications necessary for having access to the Solo feature when in Modern Mode. I plan to tweak the preamp and add an internal switch and thus be able to choose between the factory Modern+Solo > Vintage setup and the Modern + Solo > higher level Modern.
A schematics for the Modern + Solo mod would be highly appreciated. Thanks a lot guys!
Florin - Romania."
And their reply:
"Hello there,
This update is fairly straight forward. Please see the attached document.
Best Regards,
Marcus Daniel | Customer Service"
And here's the attached document: http://dl.dropbox.com/u/50540628/soloMod.pdf
You're welcome.