http://www.romana-hamburg.de/cds.htm (http://www.romana-hamburg.de/cds.htm)
If I understand this correctly, Mr Helzel has produced all these CDs with MIDI sounds (Garritan Personal Orchestra.)
I must confess, there a some of whom I have never heard before...
Kleffel (1840-1913) I have heard of but never heard and I think perhaps never seen anything by (though IMSLP has some works by him, including his string quartet -see here (http://imslp.org/wiki/Category:Kleffel,_Arno). I see my name in the edit history of the string quartet, so ok, I've seen that... :) )
Walter Niemann (1876-1953) wrote a lot of piano music and other works I've seen at Sibley and IMSLP (see here (http://imslp.org/wiki/Category:Niemann,_Walter) where there is also a list of his works.)
Of Ferdinand Pfohl I see from MusicSack that his dates were 1862-1949 (or perhaps 1863- ) and from IMSLP that he was the dedicatee of works by Niemann and JB Foerster.
Hugo Kaun has been the subject of this that and the other here before- there's even an (electronic) performance of a piano concerto of his (reduced) at IMSLP. Helzel has put up performances before of Kaun's symphony no.1 and of other works, I think.
Regarding Heinrich Sthamer (born 1885 Hamburg, died 1955 according to de-Wikipedia (http://de.wikipedia.org/wiki/Heinrich_Sthamer) and Mr. Helzel though MusicSack seems unconvinced about the deathdate... ) (actually , at IMSLP we have his Anna (http://imslp.org/wiki/Anna,_Op.8_(Sthamer,_Heinrich)) in a reduced form.)
All I know about Hugo Rüter was that he was a friend of the cellist August Nölck.
I'd forgotten about this site. The main thing that interested me was volume 3 of Walter Niemann - unlike the others this contains recordings actually made by Niemann - if it wasn't so darned expensive (more than the price of another CD) to send a cheque abroad I'd order this one.
Rob