Marantz pm8006 vs Denon PMA-1700NE for combined home theatre and music room

but I think bi-amping is unnecessary 

You are telling the OP to get an integrated amp by Yamaha, even Yamaha says you should bi-amp:

THE PHYSICS BEHIND IT

When an electrical current (i.e., audio signal) flows through a speaker wire, a small magnetic field is generated. Conversely, when a magnetic field is passed through a wire, an electrical current is generated.

Low frequencies such as the sound of a bass drum need large amounts of current to move the woofer in the speaker and push air across the room (thus producing sound waves we can hear). The signals that are sent to the tweeters in speakers (such as, for example, the “ting” of a triangle) require much less electrical current to make the small dome of the tweeter move. Nonetheless, both cause magnetic fields to be generated in the wire.

The problem is this: If that thud of a bass drum and the ting of a triangle are sent down the same wire at the same time, there is potential for audio degradation of the sound of the triangle. That’s because high frequency signals (with their relatively low currents of electricity) are susceptible to being unduly influenced by the higher-current low frequency signals (and their associated magnetic fields) simultaneously being sent through the same wire.

The solution is to separate the two signals via bi-amping — something that greatly reduces interactions between the two signals. The internal crossover network in the speaker restricts low frequencies from traveling through the high-frequency wire, thus smoothing out the signal path for the tweeter. It’s like driving on a newly paved road versus a dirt road. You’ll get there either way, but traveling on the paved road will get you there in better shape.

By the way, you may think that bi-amping seems like a good way to make your speakers louder. After all, two 100-watt amplifiers powering a speaker should make it sound twice as loud, right? Wrong. Thanks to the laws of physics (which state that there’s an exponential relationship between power and loudness), in order to hear something “sound” twice as loud, a speaker needs to receive ten times the power. Simply the doubling the power only produces a small increase in level, so bi-amping really doesn’t make the sound any louder … but it definitely makes it better, with cleaner, solid bass and more detailed highs.

https://hub.yamaha.com/audio/a-how-to/how-and-why-to-bi-amp-your-speakers/

@kota1 Sure, all else being equal bi-amping is nice but obviously costs more, and remember this is the OP’s second system so he may or may not feel the need to throw the works into this mixed-use system when he already has a dedicated 2-channel system for more serious listening. Between us he’s got two good options so really comes down to what his priorities and budget are.

The cost is actually surprising, that Rotel 6 channel amp costs $500-$600 on the used market, its less than the receiver! 

I understand your dilemma as I've been through the same though-processing.  I understand you already have the Denon.  Would it be worth it though, in elegance or simplification, to sell it, if you don't feel it's good enough, and simply get a surround receiver that also plays well in two-channel mode?  There are plenty that do so.  Cut the number of boxes, cables, complications...  

@cainullah

Been there before and the best solution is to keep HT and two channel completely separated, except for one interconnect between the L/R pre outs of the Denon and the HT bypass inputs of whatever integrated you buy. This assumes that the AVR can output the front left and right channels separately and the integrated amplifier has a HT Bypass option.

This site should help.

http://www.audiophile.no/en/articles-tests-reviews/item/426-amplifiers-with-processor-input