all off the top of my head…
Beethoven was evidently unpleasant to be around, but he was in his own way quite devoted to temporal matters. When his brother died he took custody of his nephew Karl, and apparently took this charge very seriously. So seriously that Karl later attempted suicide (apparently B’s attentions weren’t appreciated). The deepest insight into Beethoven’s personality might be found in his “Heiligenstadt Testament,” which he wrote shortly after learning he was going deaf. He grappled with the question of why Fate was tormenting him thus, and ultimately decided he had to press on and pursue his goals. Beethoven was famously absent minded. He would complain about food at taverns, forgetting what he had ordered. Once he overturned a bowl of soup on a waiter’s head. Something of a dandy in his younger days, he became more eccentric in middle age. He would wander the streets humming to himself, unkempt. Children sometimes taunted him on the street, throwing rocks etc. He was paranoid that his servants were cheating him. He exploited publishers, sometimes selling the same piece of music to two different firms. One time he was arrested as a vagrant because his appearance was so disheveled. When the police realized he was the illustrious composer Beethoven, they released him.
Chopin, we see from his letters, was a charming personality with a bawdy sense of humor. He was a snob and social climber, always looking to get into higher circles, insisting on the latest fashions in clothes, etc. He associated with Polish expatriates in Paris and his music reflected a passion for the Polish cause, but in general he avoided politics and didn’t show a broad range of interests beyond music. He shamelessly used his friends – particularly his hapless factotum Julian Fontana, whom he sent endlessly on errands to find him apartments, get him better servants, better clothes, etc. Chopin was probably most at peace in the summers at Nohant, his longtime lover George Sand’s estate, in the late 1830s and early 1840s. Here he composed prolifically and participated in amateur theater with Sand and her kids. He was a skilled mime/impersonator with a wicked gift for caricaturing others. He was nervous on the concert stage and preferred to play for intimate audiences in salons. He was fashionably anti-semitic, complaining that music publishers would “Jew” him and deprive him of money. He taught piano to dilletante aristocrats in order to pay the bills. He took this job very seriously and was by all accounts a dedicated, conscientious teacher. He could occasionally fly into a rage, as when a listener told him his rubato was so extreme that he was effectively playing a 3/4 mazurka in 4/4 time. One of his closest friends was the painter Delacroix, with whom Chopin had long conversations about art and music.
Mendelssohn was a cultured, well-rounded gentleman. In addition to being a prodigious composer (writing masterpieces by the age of 16) and probably one of the 10 or so best pianists in the world, he was a highly skilled artist. I don’t know much about Mendelssohn’s bio or his personal behavior, though.
Schumann was a brilliant music critic, writing important reviews at the tender age of 20-21. He famously wrote of Chopin at that time, re Chops’ Variations on La Ci Darem La Mano, “Hats off, gentlemen, a genius!” His music is arguably more eccentric, more weird and offbeat, than anyone else in the Romantic generation of composers. He eventually went mad and (I believe) attempted suicide. He was devoted to his wife Clara, who after his death became one of Europe’s most celebrated pianists and a popularizer of his music.
Liszt was a real man of the world. Probably the all-around nicest guy of all the classical composers. He was intellectually curious, deeply religious, and tended to hang out with bohemian revolutionary types. He was a lady killer and once (according to rumor) used Chopin’s apartment for an illicit tryst – Chopin never forgave him. Liszt was generous to a fault. He bent over backwards to use his influence on behalf of lesser-known composers – Wagner being the most famous example. He donated proceeds from lucrative concerts to charitable causes (Hungarian flood relief, Beethoven monument, to name two). He toured tirelessly – in an era before railroads – and almost single-handedly created the modern piano recital. In his later years we see a wonderful portrait of him by young American piano student Amy Fay, as depicted in her book “Music Study in Germany.” She came to Germany to study around 1870 and took some master classes with Liszt. She portrays him as a kind, witty man, a generous teacher, and still something of a flirt into late middle age. Liszt had some children. He had two major relationships in his life, one with the countess Marie D’Agoult, the other with a Russian aristocrat, Princess Caroline Von Sayn-Wittgenstein. Both of these women were embroiled in other marriages and so Liszt never married, though he desperately lobbied for the Catholic Church to annul Wittegenstein’s first marriage – they refused. In later life his religious devotion increased and he became an “abbe.” He is seen as a generally sunny personality, but he also had that “diabolic” side, especially in his youth – copied from Paganini, and evident in some of his music, such as the Dante Sonata. Even in his tranquil old age he wrote some moody and very challenging music such as the harmonically forward-looking “Nuages Gris.”
Mozart is accessible through his letters. He was bawdy and humorous. Apparently a skilled billiards player. Financially inept, always borrowing money from people. His wife Constanze was his second choice (he was originally planning to marry her sister). His difficult relationship with his father, who taught him everything about music but did not approve of his marriage, is well known. There is probably a lot more to his personality (we must avoid the danger of slipping into “Amadeus” cliches) but I have read no Mozart bios so I can’t say.
From what little I know of Wagner, he was apparently a pretty shitty personality. Like Mozart or Chopin he tended to unconsciously use his friends, and he borrowed money a lot. His anti-Semitism is well known.
I know little about Bach except that he had a big family. He once walked something like 70 miles to hear Dietrich Buxtehude play the organ. He was a bit of a tech-head, capable of repairing and maintaining organs himself.
My gut feeling on this issue is that, to achieve greatness in art (and probably many other endeavors), one must be focused to a rare degree – to a degree that would exceed the “balance” the rest of us seek in our lives. Social concerns and financial concerns – and health concerns, a lot of these guys died young and it may not just be coincidence – do seem to fall by the wayside sometimes. (Edit: then again, compared to the average life expectancy of 18th/19th centuries, composers and poets may not deviate much from the norm, now that I think about it more.) A lot of these artists seem to make use of “enablers,” people who help shelter them from the real world, and whom they sometimes unconsciously exploit. I don’t have enough data to make any confident generalizations however.