Help connecting a receiver to my McIntosh MX136 preamp

You'll want to use the "External" on the Mac, and that will give you control of the 4K on the Marantz.  You can still use the stereo on the Mac ( which is hard to beat) and have all the goodies of the Marantz..

Most folks would leave the Mac out, and use just the Marantz.

I know the Mac is a better music machine by far...HT, wise, it's just too old and 4K in Mac, is pretty expensive..

MX121 or 22 at least.. 3-6k...

I don't use HT, three active front speakers just don't work unless it's a Trinaural analog processor. Then it takes a lot of getting use too..

Music and HT can work but most loose the middle speaker. That is really a must for "the best" music app.. IMO, ay...

Regards

External, is a loop for your Marantz, not any of the sources. The source(s) all of them, will stay hooked to the Marantz, you use the "External" on the Mac and they will play through the Mac.

SQ, use the Mac, just a much better preamp for music...Using TWO HT preamps and a (7 wire source) and a la te da.... you really like wire? lol You can get all the stuff to work, I just don’t understand, why TWO? HT other than you like the SQ of the Mac, and 4K. 

I just don’t see you writing, it sounds better this way or that, just, "how do I hook it up"? SQ is what your after, right?

The Mac will sound much better. The Marantz, is budget stuff, but for HT, what do you want? 4k 5k 10k, 20k...

Stereo... I came from the mono era into stereo... 4K.. LOL

Regards