Ideas for easy A/B comparison of amps ?

Not to be abrupt, but you're focusing on the wrong issue. The preamp section of your Denon will not let you fully hear the differences between good amplifiers if stereo is a priority. The preamp section of a mid-fi A/V receiver is a big ass bottleneck as you're feeding the amps a sub-optimal signal. Preamps are extremely important to what you ultimately hear from a system, so if stereo is important then consider adding a good stereo preamp into the mix along with the amp. You will probably get some benefits from adding a better amplifier, but the results will be amplified significantly with a better preamplifier. Just my experience and my opinion. Sometimes amplifiers get blamed for a lot of what's wrong with a system when the preamp can actually be a bigger source of the problem. Best of luck.

Yadda, yadda. While, in theory, all that is likely, the OP is planning on doing a comparison and he can decide for himself whether either amp is superior to what he hears directly from the built-in amp. Unless he is willing to make an additional investment in a separate preamp, this should tell him all he needs to know.

I use a Niles DPS-1 to select between amps. I had to rig up an adapter so I could connect banana plugs too it but it works great.

http://www.nilesaudio.com/product.php?prodID=DPS-1&recordID=Source%20Switching%20Systems&categoryID=Switching%20Systems&prdcdID=FG00003

Very well built and quite solid. If you need something fancier there is the Niles SPK-1

http://www.nilesaudio.com/images/PDF/SPK-1_Cutsheet.pdf

It is a bit more complex IMO.