i think it depends upon what you are looking for. america is a relatively young country. if you are looking for all that depth - of course you'll find it in places that the old masters lived.
but, take the creative juices of macdowell or deems taylor that composed a symphony to 'through the looking glass. you have poetic composers on par with schumann i think. i happen to love charles ives, too. the music matches the impressions of the poetry or song. btw, macdowell was no slouch. he studied abroad at the paris conservatory and later germany. his music was strongly influenced by liszt WHO MET THE AMERICAN! and convinced him to devote himself to composition. (the music of grieg and, to a lesser extent, schumann were also influences).
i think the reason america has many 'happy' composers too - is back in the 1800's we had such a beautiful country. unpolluted - full of trees and rivers and lakes. it was inspiring to live in. john denver was still singing of colorado when he died probably. alaska is breathtaking. a lot of artists, poets, and musicians live up there.
ok. we have mrs. amy beach - child prodigy and mathematical genius who 'puzzled out music theory on her own initiative at an early age.' her concerto in c# minor was dedicated to theresa carreno (the same teacher of macdowell) who originated in venezuela and taught in the usa.
victor herbert - irish born - but highly successful in american operettas (babes in toyland) - concertos for cello -one of which uses the cello's upper register (maybe a suggestion from dvorak?). dvorak being the most important composer to have visited the usa to that date. (1894). the principal themes of dvorak's 'new world symphony' came from his facination with folk music. in america, he explored the music of native americans and afro-americans.
if you didn't have the usa, you'd never have had 'swing' - blue's notes - a performer (rather than composer-oriented music)
stravinsky, ravel, and milhaud were a few of those classical eurpean composers greatly affected by jazz elements that came to the fore. of course, gershwin and copeland already mentioned. and, gunther schuller, john lewis, larry austin, jeorge calandrelli.
for mainstream - we have walter piston (many harmony books from that guy). he taught at harvard for 40 years. howard hanson - director of eastman school of music in rochester, ny. (organ concerto 1926 and piano concerto 1948) virgil thompson, roger sessions (princeton, uni of cal at berkely, and julliard school of music), william schuman (studied with roy harris).
cowell, cage, lukas foss, milton babbitt (philadelphia born), charles wuorinen, ben weber, david martino, elliott carter, george rochberg, john corigliano (important melodic material near the beginning and again near the end of the third movement of corigliano's clarinet concerto is quoted from giovanni gabrieli's sonata pian e forte. this takes us back to the roots of the concerto in the hands of the late sixteenth-century venetian composers who first exploited the new concertante style). so these american composers were not unaware of music history or ignorant of styles.
Topic: is this the best america can do?