Why does bi-amping speakers sound better?

You got this right. "is it that I simply doubled the power that resulted in better sound, mostly noticed the low end of the speakers was tighter, more powerful etc.... and obviously I could also play louder.  

Or is is there something about letting one amp not work as hard due to only running high frequencies while the other amp gets to just work on the low end."

It all boils down to how many circuits are executed between the source signal to the target speaker. Each circuit introduces its own noise and distortion however minor. So when the unamplified signal goes into a dedicated circuit of HF amp to HF speaker cones and the same unamplified signal goes to another dedicated circuit of LF amp and LF cone, each cone can pull as much power as it needs for the track without affecting the other circuit.

When not bi-amped, the speaker wires carries the entire load of amplified current to the speaker and the speaker filters are the only ones separating the LF current from the HF current.

Bottomline: The closer to the source you separate the current, the nicer it will sound. Nicer being subjective as some listeners do like a bit muddier tone for certain music. 

I am not a fan of bi-wiring, but bi-amping certainly has its merits.  Imagine what the power supply and power transistors are going through in an amplifier to produce full range music.  For bass transients, you have large capacitors being depleted and recharged within milliseconds, and large amounts of current rushing through the output stages for every bass note.... at the same time, that same amp is expected to deliver extreme clarity and stability in the midrange and up.  That's a tall order.    
Now imagine you could separate those jobs.  To have an amp that can focus all its resources on just the critical mids & highs, without huge rushes of current to deal with the low frequencies.  Of course it will do a better job of it.   All it takes is twice the money, twice the space, and twice the power.   Is it worth it in the end.  I dunno.  Maybe one amp, twice the price, would sound better.

http://www2.rdrop.com/users/twest/audio/index.html

As discussed extensively below, the solution for the first two issues an active vertical biamp combined with low impedance cabling. The second two issues are addressed by balanced interconnects and reducing power amplifier gain—one of the odd things about audio is preamplifiers don't actually amplify. They attenuate. Line level out of a CD player is a few hundred millivolts RMS, but typical voltages at the drivers are maybe 100 mV RMS. In order to achieve a couple hundred watts of output power, power amplifiers have voltage gains around 30 dB. The preamp therefore ends up attenuating by 50 dB, meaning RMS voltages in preamp output stages, active crossovers, and power amp input stages are hundreds of microvolts. This exacerbates pickup of ground bounce or supply ripple coupling into power amp input stages and degrades op amp performance. The fix is simple; turn the power amp down and the preamp up. In an active biamp, unity power amplifier gain generally provides sufficient SPL and increases the SNR of most parts of the signal path by 30 dB. A secondary benefit is op amps behind the preamplifer's volume control operate closer to their line level sweet spot. With current generation op amps such as the LME49740 the benefit is small compared to speaker and power stage limitations, but there can be noticeable improvement in the volume control IC or with older op amps.

 

You are doubling not only your watts, but your power supply and head room. Addind a second, identical preamp has also made a difference in my system, because I can now have a bass control without changing any phasing through crosdovers. Since my amps do have huge power supplies , I have gone from bi-amping my mains to renning the second system on nicely matched subs.

Good point on redoing the wiring first.  However if I don't buy a amp I can't listen to my stereo. 

Its a old building as as I stated, 1865 is when it was built.  It's actually a apartment above one of my funeral homes, taking advantage of free/no rent for a while.  I believe I have a fuse panel upstairs in the attic so it probably will not be that difficult to rewire.  Paint, carpet then I need to find  out where the stereo sounds best before I run new power lines to a wall.  

This thread has been informative!!!  Thanks again everyone for their $.02 :)  I am still leaning towards bi amping these speakers, just need to figure out how much power to go with.