On the McIntosh forum at audioaficionado.org there are several posts that may be of interest.
One post suggests the volume control function of the C42 is unique to that unit.
"One of the things that frustrated me with the C42 was McIntosh's optical volume control. It was Mac's first preamp to use their optically controlled volume. McIntosh chose to design the volume curve linear without the standard audio taper, which in subsequent preamplifiers this was corrected. This caused me to have to turn the volume up far past the point I was use to with all other preamps to achieve a given volume level. I am sure one can grow use to this, but I never cared for it."
Another owner gave a very positive endorsement to the C48:
"I have now lived about a month with the C48 instead of C220. i initially noticed how much quieter the C48 was. Now, after having used it for a while, i can say with great confidence that my stereo has never sounded as good! i am really happy with it, both as a pre and as a Dac."
One post suggests the volume control function of the C42 is unique to that unit.
"One of the things that frustrated me with the C42 was McIntosh's optical volume control. It was Mac's first preamp to use their optically controlled volume. McIntosh chose to design the volume curve linear without the standard audio taper, which in subsequent preamplifiers this was corrected. This caused me to have to turn the volume up far past the point I was use to with all other preamps to achieve a given volume level. I am sure one can grow use to this, but I never cared for it."
Another owner gave a very positive endorsement to the C48:
"I have now lived about a month with the C48 instead of C220. i initially noticed how much quieter the C48 was. Now, after having used it for a while, i can say with great confidence that my stereo has never sounded as good! i am really happy with it, both as a pre and as a Dac."