Trawling through Google Books today I discovered some new information on Willoughby Gaspard Weiss, son of (Jean) Gaspard Weiss, brother of Charles Nicholas Weiss, and my 4x great uncle.
The Gentleman's Magazine of 1867, volume 223, page 828, notes the following information regarding Willoughby Gaspard's son Willoughby Hunter Weiss, famous opera singer: "He was the eldest son of the late Gaspard Weiss, esq., of Liverpool and Manheim (sic)..." This is the first time I have definitely seen Willoughby Gaspard Weiss referred to as Gaspard Weiss i.e. without the "Willoughby", which Tobias Bonz and I have previously had discussions over - he believes there are references to Gaspard Weiss which refer to Willoughby Gaspard Weiss, but this is the first time I have seen it without a doubt.
The other new bit of information is the connection to Mannheim, Germany. Previously I have found references to a W G Weiss importing goods into Liverpool from Germany:
Liverpool Mercury, 7 May 1830: "1 case merchandise".
Liverpool Mercury, 15 October 1830: "1 ch [chest?] musical instruments".
Liverpool Mercury, 15 May 1835: "1 [something - I can't work out exactly what!]"
Liverpool Mercury, 7 September 1838: "1 case musical instruments".
All of the imports came through the port of Hamburg, however Mannheim could have been where he was importing them from. There were flute makers in Mannheim in the 19th century, so it is quite possible that there were flutes amongst the musical instruments that Weiss was importing. I have searched and searched but can't find anything else relating to Weiss and Mannheim. If only there was a search engine for German historical newspapers!