Why does bi-amping speakers sound better?

Perhaps monoblocks would be a better option than bi-amping? Two matching amps, driving each speaker has major improvements over a single stereo amp. Due to each having its own power supply, greater separation, and generally they are the better amps a company makes. 

When bi-amping, you've got to be careful with amp matching. If you run two amps of the same size, the bass could seem weak. If you run a larger amp on the bottom, the bass could be too strong. So, careful matching is required to be sure each half of your speaker is running at the same SPL. When you drive a speaker with one amp, the amp sees the speaker as one load, the designers have designed the crossover to level them out, to keep top and bottom at the same level.  But if each amp is seeing a different load, they can react differently, causing your sound to be too much bass, or too much treble. 

There are speakers built for bi and tri amping, with a crossover that comes before the amps, or active crossover.  So each amp only has to work with the frequencies of the speaker it is driving. And can give fantastic performance, but amp selection is highly critical. And the active crossovers usually have a volume control to help with balancing. 

Bi-amping can give amazing results! But, it can also cost a lot of money, and cause a ton of headaches trying to balance the system. Since money is always a factor, all of your money invested in one amp and one set of wires can be more cost effective than halving your monies to buy half as good amps and half as good wires. Assuming we get what we pay for. Can two $3,000 amps sound as good as one $6,000 amp? Can two $400 speaker wires sound as good as one $800 wire? Debatable. 

the bass could only seem weak if the amp runs out of power - and that is less likely when bi-amping

monoblocks & bi-amping both are expensive as pointed out above - you need more transformers for the power supply

one driver, one amp is the ultimate design - you just have to create a cost- and SQ- effective realization

not that amps are a major determinant of SQ compared to speakers, room and source recordings anyway

Thanks for the input/options everyone... it has helped however I still can't decide :)

i am actually leaning towrds trying a class d amp from d sonic.  Debating mono block, stereo or multi channel.  I did visit a local audio shop yesterday, they mentioned they had a NAD 5 channel class D amp at $4k, Anthem 5 channel A/B at $5k and a McIntosh 7 channel A/B at $6,500.  I mentioned/asked about multi channel vs stereo/monoblock and his response was since I use a AVR (Integra DHC 80.3) that I wouldn't see any benefit from mono blocks or a 2 channel amp.  If I didn't use a AVR then yes monoblock or 2 Chanel amp is much better.  

I am leaning towards trying class D due to moving into a old home, built 1865 thus not the best electrically in the place.  I worry that if I go with and amp that's A/B that it wouldn't get the juice it needs.  One of these days when money is willing I will run 1-2 20A service to where the stereo stuff is but that's going to be a while.  

Yes, my 8.1's are rated around 300w, however for a really long time I have been one who likes having more power then less power.  I prefer my ears say no well before the amp or speakers say that's to loud! For whatever the reason I have always thought it just sounds better that way feeding more power.  I also hope to get a better old infinity speaker at some point.  Possibly just the kappa 9.1's or their renaissance 80/90 or preferably something from the IRS line.  Thus trying to think ahead so whatever I buy now will have enough oomph to power whatever I get next... which will be a few years down the road. 

IMHO, you should use the money you have to upgrade your electricity. It shouldn't take more than $1K, unless you live in a Victorian manision, them maybe a little more.
Has your current electrical system been renovated recently(10-20 years)? If not, then that may be a way to significantly upgrade the performance of your components, you'll also have piece of mind knowing you aren't overloading any circuits. You can also have the electrician put in a whole house surge protector.

Viggen - 

I'm currently running a D-Sonic that is a 7channel setup w/ both front speakers bi-amped and couldn't be happier.  As far as config, you can go with whatever you specify and remember, even in a 7 channel chassis, each amp is discrete as would be a mono.  

In my system, my speakers are hard to drive and go down to 2.7ohm so I'm running 2KW (4ohm) to each speaker (nautilus 800) and compared to lower-powered A/B amps I used previously the D-Sonic takes absolute control and drives these things as they were intended.

As for AC noise/ power, you obviously know already that class D amps inherently resolve that issue and this amp is 100% dead silent.  Once you get used to the fact that class D is a nonlinear pulse wave modulated architecture with incomparable immediacy (compared to class A or A/B) I think that you will be pleased.

If you end up going the D-Sonic route, my recommendation is to get the highest output modules that fit within your budget now so you can be sure that you never outgrow the amp if you ever decide to change out your speakers.

Let us know what you end up doing.

Take care

Greg