What's the specs of those speakers, I try googling but could not find the Uni-fi 2 UF towers?
The A700 won't give you any more power output than the X4300H in real world applications.
You probably should try the AVR first and add an external amp later only if deemed necessary.
I think this is your best course of action.
At 4 ohms nominal and a low of 3.4 ohms and efficiency of 85dB, the elacs are speakers that demand more than an entry level AVR. However, the 4300 is more than an entry level AVR.
Are the center and surrounds also 4ohms with efficiency in the mid to low 80's? If so, and if you listen fairly loud and especially to sources with a lot of content going to all speakers at the same time (a typical movie does not, but there are some surround sound DVDs of concerts that can), there is a chance that you will need more than the 4300 can provide. The same is true of the A-700.
As PENG suggests, try the 4300. If the power limits are being challenged, you will hear clipping distortion on the loudest passages.
If you need external amplification, get the Outlaw 5000. It is a far more powerful amp than the Emotiva (despite having fewer channels) for the same price. I would use the Outlaw to power the front 3 channels, and the Denon for the 4 rears/surrounds. That way, nothing is have to work very hard!
It may seem counter-intuitive to buy a 5 channel amp and only use 3 channels, but the power supply (primarily the transformer) is a shared resource and sharing it among fewer channels is especially beneficial for lower impedance/efficiency speakers such as yours.
http://www.audioholics.com/amplifier-reviews/outlaw-5000