or bigger single amp?
This is a very interesting area. There are many people out there who swear that bi (or even tri) amping is the way to go - although you can't really tri-amp most MLs.
Then there are some other very notable speaker manufacturers (like Dynaudio and others) who don't give you the option - you get one pair of terminals with no option to upgrade. Their philosophy is simple - if you want more power or better sound, use a better amplifier.
I'm in agreement with the Dynaudio approach. I think that this applies especially to MLs where there is a huge impedance difference between the panel and the cone - don't forget that the x-over is designed to work with these two components. No doubt, you can get good results by bi-amping the MLs - but I suspect that you'll get unbelievably bad results if you get the amp combination wrong!!
The only real way to tell is to try it and see. Compare large single amps with smaller bi-amp configurations, and see how it sounds

.
Cheers,
David