(1) Cost. Gain matching different amps. I use two identical amps to eliminate this problem.
I can only help you with your first question. The potential disadvantage of biamping is totally messing up the decent sound you now enjoy. The differences in amplifier timing, slew rate and dynamics make biamping risky at best. If you do it, here's my opinion on the only way to do it right: Get two identical model amps - or as close as you can get - same model & serial numbers as close as possible. Then use what's called "Vertical Biamping." That is where you use one amp to drive the high & low frequency drivers of the same speaker - as opposed to "Horizontal Biamping" which is using one amp for the highs of both speakers and one for the lows. Vertical biamping has the possibilty of the least amount of timing error being introduced into the system. Timing errors introduce dynamic incompatabilities that are easily heard as a loss of total system dynamics. I have experimented with a number of setups using various equipment and have concluded that the average listener (a group in which I certainly count myself) is better off using a good quality amp, speakers, and cables and simply enjoying what you have. Remember KISS: Keep It Simple Stupid. It's as good a piece of advice in audio as it is in any other endeavor.
Hope this helps,
I disagree with Kr4 and pretty much agree with Egoss with the following comments:
1. If the speaker was not designed to be bi-amped (and most are not), but bi-wired (most cases) you do not get one of the primary benefits of biamplification: eliminating the passive crossover network. (Unless, of course, you remove the network.)
2. When the passive crossover network is removed and you setup a "true" biamped system, the differences are like night and day. Try it on one speaker only and compare the two - I think you'll be impressed. (Just beware that it may be difficult or impossible to remove the hi-pass section only of the midrange crossover if the lo-pass is on the same board.)
3. Vertical biamping is an excellent technique, but only applicable if your amps are not "dual mono" designs. The advantage is not correct timing, as was suggested, but increased power, since the higher frequencies will draw less power and each amp can "borrow" power as needed for the lower frequencies. (I'm not an electrical engineer, but I don't buy the timing issue!)
4. I see nothing wrong with using two different amps, but not with the aforementioned vertical configuration.