This is interesting. The first thing that comes to mind is that I may have wired your output pot backwards by accident. If when you turn down the pot, you lose bass, that tells me that the pot may be miswired and is presenting a LOW value load to the output cap. The first thing that goes is bass. If you find the time, can you pop the top cover and send me a couple of photos of the output jack and output pot. Please email to:
When all is correct, the rear pot should have zero affect on frequency response. It's merely a passive control right at the output jack that somewhat mimics the INPUT of a McIntosh which is also just a pot.
With the SMS Classic into a Mac, I'd recommend turning the Classic's rear knob up to about 90%. Then use the Mac's input knob/pot as your master volume control. Whether your CTP pot is wired backwards or not, at 90% that's all moot and you'll be sending the mac virtually the full signal, no bass boosting or cutting, just exactly what the preamp is doing.