Zak, I won't go into all the details to "decipher" all those specs, except for power output. If you're really interested in one of the others, I'll discuss it, but the bottom line is that for both amplifiers their numbers are good enough to provide clean, audibly transparent amplification within their designed power limits, and no sound quality difference would exist.
Now, as to those power limits, the first point to understand is that(uniquely in the audio equipment market)amplifier power ratings are specifically regulated by the FTC(Federal Trade Commission)for any amplifiers sold in the U.S., and which as a practical matter also affects those sold in Canada. Therefore in this area what you see is what you get, but you have to know what you're actually seeing. Ratings summarized in a number such as "7 x 100" mean that each channel is capable of 100 watts output, but don't generally mean that all seven channels could be driven simultaneously at the full 100 watt rating. The FTC regulations require, among other things, that the rating be with two channels simultaneously driven at the full rated power. One channel isn't permissible, but the manufacturer can spec more than two channels, including all of them. The FTC specifically rejected a proposal about three years ago to amend the regs to require "all channels driven" since this was not a realistic scenario in home listening to have all channels simultaneously running at full power. Any spec which doesn't specifically say "all channels" should therefore be understood to be with two channels at full power, whether or not that's specifically stated. You'll note that the 3800 has the full set of specifications as to its power output.
In your case, I'd estimate that a more realistic 2-channel number for the 7125 would be around 165-170 watts, for comparison to the 3800. The additional 30 watts or so would only allow a maximum sound level on brief peaks about 0.8dB higher(e.g., 107.8dB rather than 107dB, a trivial difference). Your present equipment should have more than enough clean output for safe listening levels, and there appears to be no good reason to spend the money for the 975/7125.
