I agree about the mismatch. My view (with all its biases):
PSB Stratus Mini: described as having great bass (for size)
Cambridge Audio: UK company with the "British sound" (warm)
Not having heard either of these, I'd expect an overly warm tonal characteristic that essentially hides the midrange that may be what you've described as "shut-in."
My guess is that efficiency is not really the issue though one can make some generalizations (a dangerous idea at best). That is, that lower efficiency designs tend to be more controlled hopefully with a more neutral tonal balance while higher efficiency designs let through more of the speakers inherent characteristics which includes colorations (the peaks and such mentioned before). IMHO, higher efficiency designs have a more immediate quality / quick response that provides a sense of openess (certainly not "shut in").
My inclination is to get the warmth from tubes rather than solid state components designed to sound warm and match them with high efficiency speakers that give immediacy and openess. It's just one approach. There are many others. Thinking through what direction you'd like to go with help from others at stereo shops might be useful.
PSB Stratus Mini: described as having great bass (for size)
Cambridge Audio: UK company with the "British sound" (warm)
Not having heard either of these, I'd expect an overly warm tonal characteristic that essentially hides the midrange that may be what you've described as "shut-in."
My guess is that efficiency is not really the issue though one can make some generalizations (a dangerous idea at best). That is, that lower efficiency designs tend to be more controlled hopefully with a more neutral tonal balance while higher efficiency designs let through more of the speakers inherent characteristics which includes colorations (the peaks and such mentioned before). IMHO, higher efficiency designs have a more immediate quality / quick response that provides a sense of openess (certainly not "shut in").
My inclination is to get the warmth from tubes rather than solid state components designed to sound warm and match them with high efficiency speakers that give immediacy and openess. It's just one approach. There are many others. Thinking through what direction you'd like to go with help from others at stereo shops might be useful.