Just got the Marantz NR1604 a few days ago and thought I'd give some impressions as my first post as a "thank you" for all of the insightful posts that I have read on AVS Forum.
A little background: I have had a Marantz SR7400 powering B&W CDM-9NT's in a living room for many years. I picked up the NR1604 to provide audio for a 1080p projector in a 13'x16' bedroom in an apartment. Thus, a lot of power was not necessary. In fact, at least half of its use would be to extract audio from HDMI and power Sennheiser PXC 450 headphones for streaming movies at night. Originally, this was not supposed to be a major home theater setup.
It is doing a great job passing 1080p video and powering the Senn's.
Other personal impressions that you may or may not find helpful:
Basic setup was very simple and the 1604 started passing the video to the projector while providing good audio to the headphones.
Next, it was time to have some fun. I had an SVS 20-39PCi sub from my old house in the living room, so I brought it into the bedroom to hear how it would supplement the headphones. No dice. Connecting the headphones disables the speaker outputs including the sub preamp.
To try to hear some audio aside from the headphones, I brought some small Sony surround speakers in from the living room and set up everything using Audyssey. As some have noted, the 1604 uses Audyssey MultEQ, not Audyssey MultEQ XT. This was my first time using Audyssey, and I was blown away by the ease of setup. It even informed me that I had the phase reversed on both L and R channels. The resulting sound was surprisingly good for the little Sony speakers combined with the SVS.
GUI: I personally am happy with the simple GUI. Simple graphics and basic text. It did exactly what I needed it to do without distracting me by trying to be too "pretty." There is also a smartphone app that I have yet to try out.
After a few days of watching Amazon Prime movies and TV and TED on a Roku 3, mostly using the Senn' cans, I have found the sound is good and really reflects the differences in source material.
Next: to have even more fun, I tried to hook the 1604 up to my network. Fortunately, my router is in my bedroom and I had a spare Ethernet cable that was exactly long enough to reach the 1604. I plugged in the cable and figured I'd have to configure some IP or other network stuff. Nope. To my surprise, the "Internet Radio" button immediately gave me access to a plethora of streaming stations. While streaming audio won't provide the best sonic quality, I was very happy with how easily I could find radio stations grouped by genre and location. It was also easy to set the three "Favorite Station" buttons on the remote by holding the button while on the desired station until the GUI showed it was marked as a favorite.
I have a pair of Cambridge Audio S30 speakers arriving today to replace the Sony's. I ordered these on Amazon thanks to all of the helpful reviews on AVS Forum. In fact, I auditioned some Energy and B&W bookshelf speakers at Best Buy's Magnolia department (where I bought the 1604). Fortunately, they did not have the B&W's I wanted in stock, so I went home, read your reviews of the S30's, and saved a lot of money that I originally didn't plan on spending.
A little bit of a summary:
Marantz quality, Slim profile, easy setup, Audyssey MultEQ room correction, 7.1 channel (5.1 if using Zone 2), component video inputs, Internet/Airplay streaming and updates (via Ethernet cable)
Only 50W/channel (into 8 ohms), Audyssey MultEQ has lower resolution than Audyssey MultEQ XT, no Wi-Fi
The Marantz NR1604 can satisfy a desire for a quality slim AV Receiver for a small home theater and for good quality music.
Hope that helped. I'm a little behind the times with this stuff, so other reviews may provide a better comparison to other receivers.