There is also a very clear quote from Bruckner's Seventh in the slow part of the Sixth Symphony. Šulek's use of quotations must have been documented and I shall try to find references discussing them. Here is, however, part of the tribute paid by Stanko Horvat, one of his students and himself a major Croatian composer of his generation (in a relatively traditional vein):
It seems to me worth mentioning that Maestro Šulek attained his skills as a composer almost completely alone, without a teacher to look at, or better to say to many teachers he was choosing by himself, and he chose the greatest ones: Bach, Beethoven, Brahms, Wagner,... and remained always faithful to them. The fact that an extraordinary talent looks back at past in his creation deserves special study and consideration. For me, this is a kind of anticipation of phenomena we are witnesses of today, especially in literature and fine arts, anticipation of post-modernistic phenomena and of emergence of authors “with memory”, as they say today. Master Šulek is an author “with memory”, and, as Borges and Eco write about books from past ages, so does Master Šulek speak in his scores about the music of the past. As much as we might have been surprised and puzzled by such an attitude, as we were growing older, having actively participated in the turbulent developments in music, we started to turn more and more to the past, finding beauty, peace, order and human dignity in it, all the things our unsettled age has been depriving us of.cf.
http://wam.hr/sadrzaj/us/Sulek_Horvat.pdfand another interesting survey by musicologist Eva Sedak:
http://wam.hr/sadrzaj/us/Sulek_Sedak.pdf(the latter suffering from what seems to be a "classic" distorsion from elaborate Croatian to convoluted English, but still worth reading).