My thoughts - get an AVR with pre-outs
Remember Power is logrithmic.
What kind of speakers are you getting? Specifically look at the sensitivity (efficiency) and the nominal impedance. Low senstivity speakers (80dB) require a lot more power than normal (90dB) or high efficiency (100dB) speakers. Also remember most ratings are for 8 ohms, 4 ohms requires twice the power. Some high end speakers are 4 ohms and average sensitivity and like lots and lots of power
Get an AVR that at least has pre-outs. A good power rating is 90 watts per channel or better. If that is not enough via pre-out you can always add an
external amp such as a Berhinger 2500EP to power the fronts ( normally center and surrounds have lower power requirements and most subs are powered).
Concerning bi-amping, I don't do it but if you want to give up 7.1 for 5.1 and you can separate your speakers you can power mids with one set of outputs and the tweeters with another set. You won't necessarily recreate the sound as it was originally intended and the tweeter( high) load is norammly not that much. Biwiring is similar except you connect two outputs to the same terminals; you need to be certain your AVR can handle this! Again caution and remember that the total power of a AVR is a finite amount; if you need more headroom because of the speaker impedance, senstivity or you preference for louder sound IMHO the best choice by far is an external amp.
I mentioned at first that power is logithmic, going from 100 watts to 200 watts only gives you a 3 dB increase in sound. Going from 100 watts to 140watts is only a 15% increase, maybe if you have good ears you can hear it.
The Behringer 2500EP adds 450 watts per channel (8 ohms) for 2 channels and is only $284 from B & H Photo.
http://www.bhphotovideo.com/c/product/280617-REG/Behringer_EP2500_EP2500_2_Channel_Rackmount.html
If you want more power that just 2 channels Emotiva makes a very good 5 channel amp, XPA-5 (retail $799) at 200 watts per channel or get several 2500EPs.
Good luck!