8 gauge speaker cable

I am currently using 12 gauge speaker cable in a system that contains:

Preamp...............Audio Research SP-9 Mk III
Amplifier............Sunfire Signature (1200 wpc @ 4 ohms)
Speakers.............Infinity Kappa 9 (4 ohm speakers)

Each speaker cable is 15 feet long.

I recently found a place that sells 8 gauge speaker cable. Will increasing the heft of the speaker cable from 12 to 8 gauge degrade the signal or improve it? What effect will longer runs have on the signal? Will longer runs put a strain on the amp?
It depends on the current design that you have and the design of what you want to go to. Try doing a search in the archives regarding "8 gauge speaker cable" and you should come up with a recent thread that discussed the same basic questions. Your situation might be slightly different than the one suggested there, but you should be able to make logical deductions based on the information provided there. Sean
Mitch: My only modestly educated guess would be that going from 12 to 8 AWG will not make an appreciable difference--though I emphasize that it's only a guess. To answer your other questions, longer runs and thinner wires both increase resistance, and greater resistance means greater likelihood that you'll have a distortive effect on the signal reaching your speakers. Whether that's good or bad is a matter of taste, actually. (A thin cable might roll off highs, and you may like rolled-off highs.)

More resistance will not strain your amp, at least not in the sense that it will push it toward clipping--pushing your amp to clipping would take some doing.
Unless you have a VERY long speaker run, or are doing a major upgrade in quality of the cable, it is unlikely that you will hear any difference between a 12 gauge and an 8 gauge speaker cable. If your speaker cable run is fairly short -- say, 15 feet per cable or less -- the difference in the resistance, capacitance, and impedance of the 8 gauge vs. the 12 gauge will be inaudible. (As an example, I use a 25-foot section of 12 gauge cable for the surround speakers in my HT system, and it works fine.)

The only other factor that occurs to me that might cause an audible difference is how the cable interacts with the power amp. If a problem occurs here, however, it is usually not due to the gauge, but rather a specific design configuration that causes the amp to oscillate.

To reiterate: unless you are going to a very long run of cable, or plan to substantially upgrade the quality of the cable, there isn't much reason to move to the larger size.
Actually, with a fairly resolving system you are likely to hear some difference when switching to otherwise *identical* cables one of 12 ga and one of 8 ga.

Two factors, the 8 ga will have more self inductance.
The 8 ga in a "zip cord" configuration (for example) will also have more capacitance (probably) due to the larger
diameter giving more surface area for the charge.

In other words it will have measureably different parameters. Which will sound better is another issue entirely, one that has almost nothing to do with anything that can be predicted or measured.

But, you'd probably find that there's a bigger difference between a "shotgun" wiring (even on a non bi-wired speaker) approach and standard or even paralleled wired (for the same actual speaker wires).

If you happen to have an extra set of the 12 ga wires, you can play with this and see what the differences are. Two 12 ga wires are going to get pretty close in circular mils to a single 8 ga conductor...