Hmmm. If you want the best sound for movies, you will want to use physical media, like Blu Ray which has lossless audio. Some streaming video has surround sound (even Dolby Atmos), but it's all lossy audio. Never the less, if you want to add immersive surround sound, you will need some sort of receiver or a preamp/processor (pre/pro) and separate amplifier.
Since you have a 2-channel setup already, you will need to decide if you want to replace your two channel NAD integrated amp with a reciever or pre/pro and amp, or if you want to run two seperate setups. If you replace your integrade amp with a pre/pro you might look at the NAD 758v3 since you already like NAD's sound. It has pre-outs that would allow you to run a nice two channel amp to power your stereo speakers if the 758 isn't up to par (I think it should be). Denon makes a nice product as well, and is a better value, but it might not have the two channel sound you are after. Is there anywhere I can listen to a couple of units? Personally if you use digital material, I think they will sound very similar.
If you want to run two seperate setups, then I wouldn't spend the money on the NAD758 and any of the other models you mention are fine as long as they have HDMI. You will want to run the HDMI cable from your sources (your iMac in this case) to your reciever (or pre/pro) and then another HDMI cable from your reciever to your projector. That way if you switch sources (say going from your iMac, to an Apple TV, to a Blu Ray player, to an Playstation etc) you just need one HDMI cable going to your projector and the reciever will do the switching for you.
Dolby and DTS are competing formats, that essentially perform the same thing (say Chevy vs. Ford, Toyota vs. Honda, BMW vs Mercedes).
Dolby Digital (and DTS) have up to 5.1 channels of lossy audio and were the primary audio formats on DVD and is common on TV and streaming boradcasts. Dolby Digital tops out at 640kb/s DTS tops out at slightly higher bitrate.
Dolby Digital Plus (and DTS ES) is a improved format that usually has 7.1 channels (6.1 for DTS-ES) of lossy audio but have a higer bitrate of up to 1.7 Mb/s
Dolby Digital True HD (and DTS HD-Master Audio) These formats usually have 7.1 channels of lossless audio and were the primary audio formats used on 1080p Blu Ray and HDDVD
Dolby Atmos (and DTS X) is an add on to either Dolby Digital Plus (lossy) or Dolby Digital True HD (lossless) audio. This add on adds additonal challels (normally hieght chanels) This is found on most all 4K UHD Blu Ray and in a few 1080p Blu Ray discs in lossless format. You also see it in some streaming content in lossy format. This type of system adds the number of hight channels after the subwoofer channel (example 7.1.4 in this example the sytem has seven main channels (front right, front left, center, surround right, surround left, back left, back right), the subwoofer channel, and four hieght channels.
7.4.4 Theater Room: JVC-RS500, Silver Ticket AT 2.35:1 142”, Onkyo RZ830, Anthem PVA-7, Panasonic DBT-110, JBL Studio 530’s, 4 - 15" DIY Sealed Subs
3.1 Living Room: Samsung 64” F8500 Plasma, Anthem MRX 300, Dynaudio Audience 52’s, Dynaudio Audience 122C, NHT SubOne