In my hands was the first original Hilgen schematic I have ever seen. It appears to depict the circuits of both the Basso Profundo and the Model B2503 Basso Grande. The owners warned me that there was a small gap in the middle of the schematic, but given the fact that (a) there is a comprehensive parts list on the schematic, (b) there are some logical inferences about what would be shown in the missing section, and (c) I have two examples of both the Profundo and the Basso Grande to inspect for direct evidence of what is located in the circuit at that point, the missing information should be capable of being recreated with substantial certainty.
I will be processing the image so as to increase its clarity before I post it in the schematic section of this site, but there's nothing like seeing the original appearance of an old, historic document.
I'd like to express my deep gratitude to Walter Farley and Candace Maxwell of Cross City, Florida for their generosity and patience in making it possible for me to obtain this amp and the schematic within it. Shipping the amp from Florida to Maine was a difficult and delicate operation. You won't find better people than Candace and Walter. Go ahead and try.