I have found that in The Music Trade Review dated June 23, 1900 (vol. 30, no. 25, p. 20), there is a brief article on "Sousa in Berlin" that mentions "the German Times, a weekly newspaper printed in English and circlulating throughout . . . the European continent." In it, Sousa's success in Berlin is confirmed, but there is no specific word about the Sousaphone.
But then I found the following translated quote from the paper Nachrichten, published in Dresden, dated June 16, 1900, and appearing in C. G. Conn's Glimpses of Wonders and Scenes of Their Creation, which came out in 1902:
The tone color of the Sousa Band differs materially from that produced by German brass bands, due to its visible difference in constitution. This is most marked in the case of the brass instruents which give out a softer and less blaring tone than is heard from our German musicans. Mr. Sousa is himself a skillful designer of instruments, and is - for instance - the inventor of the monster helicon, which is named after him, the "Sousaphone." To this better equipment and to the masterly art with which the musicians control and subdue the volume of sound at their leader's behest, is due satisfying tone moderation of the orchestra. The result is that it is possible to listen with delight to the band's performance of compositions usually played only by string orchestra.Further, in the May 18, 1900 edition of L'Express, published in Liege, Belgium, the reporter simply says, "The cornets are clear, the tubas are enormous."
And in LePetit Bieu on that same day, published in Bruxelles, we find this hilarious comment: "The background bristles with huge transatlantic smoke stacks which could easily swallow the musician or musicians entrusted with their operation." I assume that is a reference to the bass section, which included Conn's "Monster Sousaphone"!
I hope to come across more references in the future, but this is enough to confirm that the Sousaphone indeed caught the interest and admiration of Europeans in 1900.
But before leaving this topic (for now), I was shocked to see the following photo, posted by someone on the tuba forum website "TubeNet":
This is supposedly showing a "Czech Brass Band in Russia (end of 19.c.)," but check out the back row. Could it be that Europe actually had something close to a Sousaphone before Sousa arrived in 1900?