I am using a Hafler XL280 amplifier for the front channels, driving my Vandersteen 1C's. I've always loved the sound of this amp, but had retired it when I was using the 605 (no pre-outs.) The 1C's are a pretty easy load, and the 706 could drive them fine, but the Hafler has great bass control and a very neutral midrange. And, I've yet to hear a receiver that has highs equal to good separates.
One benefit to this is that the receiver won't run nearly as hot since it doesn't have to drive those channels.
The question of how much improvement is debatable. I can't put a quantitative number to it, but I haven't experienced a receiver that sounds effortless like a power amp, and can drive a 2 ohm load if necessary.
BTW, I've seen a number of Hafler amps on ebay, and would wholeheartedly recommend them (assuming they work, of course!) They've always been a great value, but when found used, they're a steal. Getting a pre-Rockford Fosgate Hafler amp would be my recommendation. The XL, SE and DH series amps would fall into this category (though I believe Rockford acquired Hafler while the XL and SE's were still made.) I really don't have any experience with the Rockford/ Hafler amps; Transnova series, etc.