Voltage Gain

I'm considering using two different amps in my system. One concern pointed out in the Linn website is the difference in voltage gain between amps. How much of a difference is too much difference? I'm considering using a Linn amp with a gain of 28.6 dB with an amp with a gain of 29dB. Is this material?

Perhaps (how's that for a worthless comment?). For a given load and input level, the 29 dB amp will deliver .4 dB more power. It is not ideal, but nothing in audio is. You are going to get greater variations in power delivered to your listening position through the speakers interaction with the room than .4 dB. A slight change in toe in will cause that much change. I assume you are biamping? I wouldn't worry about it as long as you use one amp for the lower frequencies and the other for the upper. Then swap them and see which sounds better.
What manner will the two amps be used? Are they 2 stereo amps to be used in a biamp arrangement? Or 2 monoblock amps one for each channel? My experience with active biamping is that such a small difference (~0.5dB) is negligible, especially if the crossover point is not in the midrange, or if the electronic crossover unit has level adjustments. For 2 monoblocks, even a small difference in gain might be noticeable (in good systems) as a slight L or R emphasis, which could be corrected by a balance control on the preamp.
those two figures are so close (0.4dB apart) that you'll hardly notice any difference at all. go for it
As test reports in the various audio rags clearly show a manufacturers published figure for gain is not always the actual gain of the amp. The situation in the real world often is quite different than on paper yeilding a significantly greater imbalance than you think. If you have an imbalance, you might try putting a Radio Shack potentiometer in a box and using it at the inputs of the more sensitive amp. Creek makes a product functionally similar to this but it is more spendy.