If there genuinely is an audible difference, it will almost certainly have to be attributed to a difference in amplifier output impedance, and thus a difference in the extent to which the speaker's impedance peaks produce corresponding peaks in the voltage at the speaker terminals. This is what Pdxwayne was alluding to when he pointed out the difference in damping factor. While this effect is a theoretical possibility, it is fairly unlikely to be a plausible explanation because even with the higher output impedance of the Krell, the peaks in the voltage at the speaker terminals will most likely be very much below the threshold of audibility.
If you want to be taken seriously by most everyone on this forum you need to use objective measurements to demonstrate that you really did hear what you thought you heard. Very many people on this forum have ample first-hand experience with people saying that they heard a difference but were then unable to demonstrate, in a double blind setup, that they could hear what they said they heard. The skepticism that you'll encounter here, when making a claim such as the one you've made, is well-justified skepticism.
Fortunately, there is an easy way for you to demonstrate that the speaker really does sound different with the two different amplifiers. This does not require taking any acoustic measurements. Because only one speaker need be used to test this hypothesis, all you need do is measure the voltage at the speaker terminals using a good voltmeter capable of accurate rms readings as high in frequency as 20 kHz. You obviously need an accurate source of sinusoidal tones, but you can find this on the web if you do not have a test CD with accurately recorded tones. You want to accurately measure voltage at a frequency that will be good for level-matching the two amplifiers, and for this, you want to use a frequency where the speaker impedance curve is very flat. Looking at the graph of the speaker impedance, 100 Hz or 125 Hz should be ideal for this. Read the voltage at this frequency as accurately as possible for one amplifier, and use this to adjust the other amplifier's output, so that at this frequency the voltage at the speaker terminals is the same for both amplifiers. Then with each speaker, take measurements at 1/3 octave intervals, or 1/2 octave intervals if it isn't fun. And take measurements at the specific frequencies where you see peaks and dips in the speaker's impedance curve. Then compare the two. If you see that voltage is different for the two speakers at some specific frequency, you can express this difference in decibels by dividing one of the voltage values into the other, then taking the base 10 log, then multiplying by 20. (Or you can express the peak as a deviation in decibels compared to the nominal value for that same amplifier, by dividing the reading at the peak or dip by the average value of the voltage readings for that amplifier.)
It will be a good idea for the speaker and the amplifiers to all be thoroughly warmed up before you do this. For example, if the speaker voice coil is much warmer when measuring for one amplifier than for the other one, this will affect the resistance of the coil which will affect the behavior the crossover and potentially affect the frequency response, especially near the crossover frequency. In fact, if you do measure a difference in frequency response for the two amplifiers, you will want to rule out this possible source of error, and to do that, you'll have to start over and make absolutely certain that everything is thoroughly warmed up when you take the measurements.
This is not a difficult to thing, and if you are convinced that the two amplifiers sound different and you want to know why, this is what you need to do. It is a productive thing to do. Much more productive than going to a web forum and reporting what you think you heard and then asking other people who weren't there to opine on the reason or on the veracity of the claim. People do this kind of thing all the time, but it serves no genuine purpose that I know of. Do the measurements, and if you find that the two amplifiers really are different with respect to frequency response, by all means report back. Meanwhile, don't laugh out loud when people express their skepticism. They have very good reason for the skepticism.
