That's the key to the kingdom. I submit that the sound of both are close enough that it will be damn near impossible to make a comparison of the two unless they are sitting side by side with the same connected material and the same media. Even then their slightly different placement will have it's effect. Therefore, any comparison based on one's "memory" of what a speaker sounds like becomes very, very subjective.
I recently got a brief listen to a brand new (not broken in) pair of Summit X hooked up to equipment that is very different than mine. I would really be hard pressed to compare it at all to my Summits that I listen to daily. Differring source equipment can and will make two identical speakers sound different.
Additionally, it would be hard to beat the Summit factoring in the current price point of $6-7K. You know, even my Vistas sound good when they are not in the same room as the higher end models. I think the people with Summits and Spires should be very happy, and unless you need to have the latest panel / woofer crossover technology and a few grand lying around, you can convince yourself to go either way.
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