I owned a Marantz 7 for a time during the early 1990s, and as its reputation suggests it is a wonderful sounding and classic piece. However assuming deep bass extension is a concern I would not recommend it for use with power amps having input impedances of less than 100K, and even that is less than ideal. As can be seen in schematics that are available at HiFiEngine.com and elsewhere its cathode follower output stage is coupled to the output through a 0.22 uf capacitor, which will result in an output impedance at 20 Hz of around 37,000 ohms!
Also, I’ll mention that I found the sonics of a pair of mono Marantz 1s I also owned at around that time to be a bit richer and slightly preferable to the sonics of the 7. Although condition and/or the particular tubes that were used in each might have been factors in that.
I have no experience with the C22 or the SP10. However the C22’s output coupling capacitor is also on the small side, at 0.47 uf, and although its manual recommends a minimum load impedance of 50K I would again suggest 100K or more, assuming deep bass extension is a concern. The output impedance at 20 Hz in this case would be around 18,000 ohms.
The SP10, on the other hand, appears to use a parallel combination of 10 uf and 0.015 uf in series with its outputs, and consequently has a minimum load recommendation of only 20K. Although I would have significant concern about the sonic quality of a capacitor that is as large as 10 uf and is directly in the signal path, and even more so considering that it is now more than 30 years old.
Good luck. Regards,
-- Al
