Most specs, including the ones you have cited, have little if any relation to sound quality, and should not be the basis of a choice between high quality components. Since you can't audition the Simaudio's at home, I would suggest simply researching reviews and user comments as thoroughly as possible, and if possible auditioning at a dealership.
I am not particularly familiar with either make, but I looked over the specs and literature on the W-7 and P-7 at the Simaudio site, and I looked at the very limited information which is available at the Cairn site.
The main thing which struck me is that the W-7 claims it is a no feedback design (in the manual; the brochure claims "no global feedback," which leaves the door open to local feedback around individual stages), yet its spec on output impedance (and damping factor, which is output impedance divided into 8 ohms) is exceptionally low (high in the case of damping factor). The lower the better, everything else being equal, but usually such good numbers are obtained by using large amounts of feedback, with its possible adverse side-effects. Harmonic distortion numbers, which are also helped by feedback, are also significantly better than average for a no-feedback or low-feedback design. It would be worthwhile to gain an understanding of what their "Lynx technology," which is claimed to make all this possible, actually does.
The W-7 has excellent peak and continuous output current ratings, and doubles its output power into 4 ohms, relative to the 8 ohm rating. I'm not familiar with your speakers, but if they are rated at or near 4 ohms, and/or if their impedance dips to low values at certain frequencies, that will be beneficial.
The only other spec I find notable is the Cairn preamp's 50 mv input sensitivity, which is more sensitive (a lower number) than usual. That conceivably could present a gain mismatch with some amplifiers (correctable with an attenuator pad in between), but since you would be replacing both the preamp and amp together that is not really relevant.
One minor correction: The upper end of the frequency response ranges you listed should be "kHz," not "Hz."
Hope that helps,
-- Al
