Itching for better sound

I would go for the Yamaha, it has the most power of the three and the lowest price. The Marantz seems a little under powered. The Cambridge does not have tone controls which may or may not be an issue for you. Of course the sound quality is the most important and I have only heard the Yamaha and it seems to be a pretty nice sounding amp.

My A-S801 really makes pianos sing - and the inbuilt DAC really pumps up the value/performance ratio. 

D'yall think my non-audiophile ears are going to hear a dramatic difference between my current Denon AVR1404 and the Yamaha A-S801?

Short answer, yes you should hear a significant improvement by swapping the AVR for an integrated stereo amp. But I have to ask, why do you need two sets of speaker output terminals?

Anyway, with the Denon you’re not only using a compromised amp but also, at least as importantly, a compromised preamp. By compromised I mean that they’re designed to meet a competitive price point, so expensive yet crucial electrical components — such as power supplies, etc. — are usually of lower quality and significantly affect the quality of the resulting sound reproduction. The amp/pre sections are also bombarded by a litany of other noisy electronics necessary for an AVR to do all the things it needs to do (DAC, digital signal processing, etc.) as they all reside in the same case. So by adding a dedicated integrated stereo amp you get higher quality components, more focused and accomplished circuitry and design, and less inherent noise you’re likely to hear a notable improvement, especially as your speakers are refined and revealing enough to convey such differences. Things such as bass weight and definition, cleaner and extended treble detail and air, image separation and solidity, soundstage width/depth/height, ability for sound to separate from the physical speakers themselves, tonal richness and shadings, micro/macro dynamics, as well as overall transparency should all improve markedly. And when you combine all these improvements it results in a totally different and more believable, involving, and more rewarding listening experience.

Lastly, one of the reasons I recommended the Sphinx is that it comes from a very highly regarded designer/manufacturer with an impeccable reputation for service, and it benefits from the trickle down of technology and design implementation from its more expensive siblings in the Rogue lineup. And although it’ll sound great with its stock input tube, you have the flexibility to change the inexpensive tube to custom tailor the sound to your specific liking. Moreover, it’s good enough to reward you for any speaker upgrades you make in the future, and as the Sphinx doubles its power output to 200Wpc at 4 Ohms impedance it has the juice to easily drive just about any speaker you throw at it. But, amps are still a personal choice based on individual personal tastes, so I thought this excerpt from a CNET review of the Yammy A-S801 might help put things in perspective and help with your decision...

“While the A-S801’s competence was never in doubt it lacks the more "robust" character of the $1,295 Rogue Sphinx amp’s sound. The Sphinx is a cool running vacuum tube/Class D hybrid design that sounds like a classic 1980s high-end integrated amplifier. The Sphinx is rated at 100 watts per channel for 8 ohm speakers, 200 watts per for 4 ohm speakers. There’s more body and soul to the Sphinx sound, the A-S801 is tonally cooler and lighter on its feet. Which one is better? I prefer the Sphinx, but tastes vary, you might go for the A-S801.”

Hope this helps, and best of luck.

Thanks so much for that thorough explanation.  As a non-audiophile, it's hard for me to figure out how amp and not speakers will make the biggest difference, but I've listened to all the people here.

For a number of reasons, I often like to have the same music playing in two rooms, hence the two sets of speakers.