I did find the information that my husband passes around to his class (everyone say thank you to Erik, lol). It is not an Intrumentalist article as I thought, but rather in a book called "Music for Concert Band".
I want to make sure I give all the credit to the author and say that this is just an excerpt for a sort of educational use, because I don't want to be breaking any copyright.
This is taken from the introduction to "Music for Concert Band" by Joseph Kreines, published by Florida Music Service, copyright 1989.
" EASY - (grades 1-2 approx) - Basic rhythms, techinically limited with simple textures, effective doublings for ensemble security, limited solos.
MEDIUM EASY - (gr. 3) More elaborate rhythms, increasing technical facility, expanded ranges, greater techinical independence, more solo and small-choir scoring.
MEDIUM - (gr.4) Varied rhythms, expanded technical demands, more complex harmonic and contrapuntal content, metric variety, greater range of keys, more musical and scoring subtleties.
MEDIUM ADVANCED - (gr.5 ) More substantial musical and technical requirements, maturity of tonal, rhythmic and stylistic concepts, soloistic capabilities.
ADVANCED - (gr.6) Fully developed musical and technical ranges, including complex rhythms and meters, intricacies of articulations, full dynamic spectrum, full solo and section capability."
So what I take out of this to apply it to solo music is just remove references to ensemble playing. However, there is still that to take into account, like in the Hindemith Sonata for Bassoon and Piano, 2nd movement, the bassoonist has to play independantly of the piano, rather than a piece where the piano plays the melody with the soloist or something like that. Also, the way he words this, you could add a grade 7 into it if a particular state includes that.
Hope this helps!