Guillaume Ferdinand Teissier, Essai philologique sur les commencements de la typographie à Metz (Metz 1828), pp. 144-45 (link).
"Msr. Lyon-Asser was promoted to the much-sought after position of Chief Rabbi of the Jewish community of Metz, on the reputation of one of his works titled Schagatharié (the lion's roar), a folio volume printed in Germany. I acknowledge that I am unqualified to pass judgment on such a work. According to the testimony of our learned Hebraists, especially of Mr. Gerson Levy, considered very astute in Hebrew literature, this book, full of the subtleties and controversies that make the base and seasoning of Rabbinic scholarship, demonstrates that had Asser Lyon chose to train for instruction in another career, he would have made a name in the literary world because of his elevated mind.
"He died in 1784, leaving a son who is today the rabbi of Karlsruhe, in the Grand Duchy of Baden, and whose name is still dear in Metz. The scholarship of Asser's son has not been merely to fathom the depth of rabbinic scholasticism, to seek clarity amid the darkness of the Talmud, the Midrashim and Zohar - it is learned and literate in our own way; to whit, he was the friend and rival our learned countrymen [Moses] Ensheim, who now lives in Bayonne, and Isaiah-Berr-Bing, who died twenty years ago, who was the administrateur général of Salines de l'Est.
"To conclude this brief notice of Lyon Asser, allow me to link a name dear to France with the Metz rabbi. The Comte de Provence (Louis XVIII), during his stay at Metz in 1782, went to visit the synagogue, where he attended the Friday evening service. He was received by the trustees, headed by Lyon Asser, a venerable looking old man, whose noble and serious mien recalled the idea of a priest of the old law (i.e., a biblical kohen).
"The prince received the tributes of the Israelites and did not despise the blessing of the chief rabbi, who presented him with a Bible written on a scroll. The memory of this old minister remained in his memory; one was surprised to see praises of a Jew coming from his mouth. "Jew or Christian, what do I care," said the brother of Louis XVI to me, whose virtue I honor wherever it presents itself. These words were not wasted. Judge if the Jews of Metz saw this same prince happily return to the throne of his fathers!"
In case it isn't clear, Lyon Asser is the Shaagas Aryeh. Lyon = Leib, or Aryeh, and Asser = Asher, his father's name. His son was known as Asser Lyon. For a time he was the rabbi of Wallerstein, and he took that as his surname. Eventually he became rabbi of Karlsruhe, as mentioned in this excerpt, and he was about 74 at the time of this writing, 1828. It's interesting that it says that he is remembered well in Metz, because according to Jay Berkovitz (The shaping of Jewish identity in nineteenth-century France p. 95) he was going to become the rabbi of Metz (actually, the consistorial grand rabbi), but then accepted a different position "dismay[ing]" and "anger[ing]" communal leaders.
ETA: This is an image of the Shaagat Aryeh made after his death, from his actual body as it lay on a bed. It is supposedly authentic -
The image is reproduced on pg. 134 of Nathan Netter's Vingt Siècles d'Histoire d'une Communauté juive (Metz et son grand passé) (Paris 1938).
There, Rabbi Netter writes that he learned of the image's existence from R. Isaac Herzog in Dublin (he describes him as the successor to R. Kook in Jerusalem, which indeed he was then). He was able to obtain this very image through Esther Herzog-Goldberg of Paris, who I guess was his sister. She explained the origin of the image as follows. Their father, R. Joel Leib Herzog, who was a rabbi in Paris, met Louis Bloch, who was a sixth generation descendent of the Shaagas Aryeh. Rabbi Herzog asked him if the family had any portrait of him, and to his surprise he was informed by Bloch that his sister owned an oil painting of the Shaagas Aryeh, executed while he lay on his deathbed! Rabbi Herzog paid to have a reproduction made, and this is it. The section of the book concludes with a well-known story which Esther Goldberg told him, about how the Shaagas Aryeh was already aged - nearly seventy - when the community of Metz appointed him. He came highly recommended, and his reputation, because of his work Shaagat Aryeh, was high indeed. However, when they met him he appeared quite old, and this concerned them. He told them that he realizes they think he is too old; how long do they want their rabbi for? They replied "twenty years." He assured them that he would be their rabbi for twenty years, with God's help - and he was.
And here is the Lion Asser's son, Asser Lion (R. Asher Wallerstein) mentioned in the post, from the same book: