More Small and Smallish prologic and prologicII processors.
Prologic does work for scAtmos but I found prologic II to be a bit cleaner. The matrixing should be and did seem more precise.
If you are on a budget then either works.
Audiosource SS Three/Four
The audiosource SS Three/Four are simple prologic processors from ~1990 that have surround speaker amplifiers only (which are useless for scAtmos)
The SS Three is actually a higher quality unit than the SS Four. It has a better circuit board and a toroid transformer that hums a lot less than the pair of EI transformers in the SS Four (although I was able to cut the power to the larger amplifier EI transformer in the SS Four and the rest of the unit did not need it)
Prologic processor from Onkyo that has a 50w center channel amplifier built in. This is a well built older unit from a reputable AV supplier.
It would still require 4 channels of amplification for the Front and rear on each side but it does save something... It doesn't have a center channel preout so it would have to use the internal 50w amplifier for the center channels but given that it is rated at 50W into 8 ohms @ .09 THD it should do the job.
(requires A/D converters): (had a pair of these)
I actually liked these. I had no real issues with them and they worked just fine but they don't have the extra control of prologic II music mode.
(requires A/D converters):
I went with the SH-AC500D instead and didn't try these. I guess I just hated the Tan faceplate too much...
Prologic II Processors:
The original and current recommendation for scAtmos is to run prologicII decoders. While the original prologic works well it doesn't have the advanced features of "music mode" that allow for adjustment of the center channel width.
Tritton Pro+ Headset decoders:
Cheapest little box I found! 6.5" wide, 5.5" deep, 1.5" tall (they even come in black, white, red, and silver) They save rack space but then require 6 channels of external amplification which kind of cancels out the savings...
They have to be models 90303 or 90302 to have the 3.5mm 5.1 analog outputs but they seem to show up on ebay pretty regularly and I even found one at goodwill for 3 bucks! Lesser models don't have the analog inputs.
They require an external Analog to digital optical converter ($15 or less on amazon) and a 9V power supply.
They use 3.5mm stereo outputs so you have to buy some 3.5mm to rca adapters (or cables) but it isn't that big of a deal.
Other than that they plug in and work. They do have volume control but it is fairly basic, essentially you can turn down the volume on the center relative to the L/R. They also don't have any control over movie or music mode. It appeared to be only movie mode based on my testing.
They were much better than the HD Audio Rush boxes in my experience with MUCH less noise.
There was some noise on mine but it really seemed like it was from the A/D conversion because if I unplugged the digital input from the boxes the noise was drastically reduced. A better A/D converter might have helped.
I did actually try a set of the much older Tritton AX360 prologic modules just for fun which also worked....
Rotel RSP-1066 and RSP-1068:
Smallish Prologic II preprocessors, They were surely top of the line in their day.
I almost bought one but they were over $100 on ebay and actually larger than a marantz slimline and also require 6 channels of external amplification so it wouldn't save any rack space.
They also don't have modern room correction.
The winner for me was:
Marantz Slimline NR140x/NR150x/NR160x:
Much more modern, the NR140x units are 5.1 and the NR160X units are 7.1. Most of the NR150x units are 5.1 but some are 7.1.
I recommend getting a NR1403/NR1603 or newer model used on ebay. The NR1501s work and are often found for a good price but run too hot to stack.
The NR1X03 and newer models are also fully compatible with the newer Marantz remotes. While this might seem like a bad thing it is easy to disable the IR sensor for normal usage and turn it back on to configure them. It is nicer than having to search for a remote to configure them if you want to re-run audyssey or play around with prologic II music settings.
I found the NR1403s to be the most common and cheapest of the slimlines that met my scAtmos needs.
They have HDMI onscreen setup, Audyssey MultEQ, 50w/channel amplifiers and are only 4" tall.
The menus on the NR1403 use older looking on screen display graphics compared to the NR1603 but they are really only needed for audyssey so it doesn't matter at all.
The 5.1 receiver was just fine for me where the 7.1 just had more potential to get hot from idle amplifiers running. I also like the extra empty space in the NR1403 where I added a rear exit 80mm PWM controlled computer fan.
Below are some pictures of my modified nr1403 where I added heatsinks to the hdmi board chips, a 4 wire temperature sensitive pwm fan controller, and an 80mm pwm fan with a decent grill to cover it. The airflow out of the top of the AVR from the HDMI board was much more restricted when stacking them than the amplifier heat sinks were. The main amplifier heat sinks are open to the top and bottom so stacking two aligned units still allows the air to flow freely through the amplifier boards. I added the fan so that I can stack them 2 high and not worry about it. The fans now pull air from inside the unit and out the back. The temperature sensor is on one of the heat sinks and if it were to get warm the fan will speed up and move more air. Setting the fan controllers to a low start speed where it ramps the fan speed quickly doesn't seem to ever cause the fans to speed up so they run quietly.
The fan controller is a simple ebay unit that costs about $4 shipped from China. I used an all plastic velcro on both it and a spot on the board where there were no components. I then used a 3" hole saw to cut the hole for the 80mm fan and drilled holes for the mounting screws. I then also drilled a small hole for an added 12v DC input jack to power the fan.
The nr1403s in my setup are running prologic II exclusively they really can't have more than 2 channels driven at a peak of 2x50W at a time. Even if they have all 3 channels running it is just matrixing the sound intended for 2 speakers so the spl and power used shouldn't increase. I have not noticed any heat issues even after running a full atmos movie at ~72 on the sr7010's.
Compact class D amplified current model pioneer.
I haven't seen one in person but I like how small they are and the class D amplifiers explain how.
If you want a new compact unit this should fit the bill!