When your preamp doesn't match your amp


From what I've gathered, there are some great amps (e.g. Pass' Aleph) which are quite finicky about the preamp they're connected with. I understand this is typically tied to an impedance mismatch, but what I don't understand is what does this *sound* like?

Could someone describe the symptoms of a poor preamp/amp connection?

Thanks in advance,
mprime
I had a Sunfire Reference Preamp in my system for about a month. I had it matched with A B&k Ref 4420. Voices were hard sounding, highs seemed to roll off, transients seemed soft. I was also told that perhaps the specs were not a good match for the B&K. I put a PT3 MK2 from B&K back in my system with much smoother rsults.
I'll defer to the more technically-oriented, but I believe a major symptom of an impedance mismatch is a rolling off of the highs.
In some cases buzzing from your speakers. GamuT amps have a piano switch for lack of a better term that allows you to match the impedance of your preamp.