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,
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.
Normally the amps will sound slow, not really dull, but slow and they have no air in the high frequencies. Unforutnately you will only notice it, when you know the reproduction when everything is right.