"Should I bi-amp my speakers. I have extra channels in my AV receiver that aren't doing anything. Is it a good idea?"
The short answer is "no, you shouldn't."
Just to clarify, here we will talk about passive bi-amping, where the existing passive crossover remains and one channel drives the tweeter while the other drives the mid. This is different to active bi-amping, in which an active crossover is used and replaces a passive crossover. Each driver has it's own amplifier and the crossover is inserted at line level before the amplifiers. This is a good idea if done correctly, but requires measurement tools to implement.
Reason no.1 - there is no advantage
No extra power is delivered, since the passive crossover remains in the signal chain. While your receiver might in fact deliver 2 x 100w instead of 1 x 100w to each speaker, the extra 100w is still lost in the passive crossover. One channel feeds the tweeter, and about 85% of the power that would normally have gone to the mid is filtered out. The other channel feeds the mid, and about 15% of the power that would have gone to the tweeter is filtered out. The net result is no different.
Some who have misunderstood passive bi-amping, may claim that clipping can be avoided. This is an advantage of active systems, but with passive bi-amping, both amps if identical will clip at the same time.
There are no magical benefits to passive bi-amping and there are no real ones either.
Reason no.2 - there is a downside
The extra channels in your AV receiver that are not used are not wasted. Affordable receivers are not well designed to deliver a continuous signal to all channels at the same time, the power supplies and heatsinks are simply not up to the task.They are counting on surround channels having an easier task. If some of those channels are not used, that adds headroom the reduces the load. This is a good thing, not an idle waste of resources. If those channels are made to passively bi-amp the mains, they will not only use up that headroom, but they will present a heavier load than was intended for those channels. When pushed, the amp will clip sooner.
Less is more
If you don't need any of the surround channels, give your receiver a break and let them remain unused. Less in this case is definitely more. You will no doubt hear some report benefits and subjective improvements, as you will with many tweaks, snake oil or otherwise. The simple truth is that we are easily tricked, especially when we want to believe. The appeal of this tweak is that it's so easy to do, and if you've asked this question, you are probably looking for a simple easy way to get an improvement. I suggest instead that you have a look around here, and I will show you other ways to get a very real improvement. If you are new to the world of audio, then some of the improvements you can experience are startling. Let me suggest a few things that just might make your jaw drop:
- DIY subs - suddenly the sub you couldn't afford looks quite unimpressive
- Learn the tricks of integrating your sub
- Add some serious bass traps - what you thought was good bass will soon sound like rubbish in comparison and you will be surprised how good your existing sub can sound
- Consider building some good DIY speakers - the result could be a real eye opener
- Pay careful attention to speaker placement - small changes can have dramatic results
- Build some acoustic treatment - the difference is not subtle