Well, poptop, your source is different from mine. Mine is a letter to Jon and Louise Webb, dated February 28, 1966, found on page 240 of Screams From the Balcony. In that letter, he says he was listening to Brahms' First.
So, typical of Buk, the story is a little bit different each time. Not that the real answer is very important, but I would suspect that the 1996 letter might be more reliable than a poem, but I suppose it depends on how drunk he was writing the letter vs. writing the poem. Or maybe it was neither, and it was just mentioned for literary effect. For all we know, he could just as well have been taking a hot, stinking beer shit and listening to the Brandenburg Concertos when the FBI showed up.
So anyway, while I am familiar with that Symphony, but don't really care for Brahms, and my opinion is entirely irrelevant, your answer is at least found in Buk's writings.
Next question's yours...