THETA DREADNAUGHT II TRIGGER

That's exactly what I'm doing, it's the only way.

The problem is not the cable, it is the Theta Dreadnaught II required 12V pulse signal instead of constant signal that your MX121 sent out.

In order to turn off the Dreadnaught II, you need to turn on the MX121 again then turn it off.

An stereo to mono adapter or a stereo to mono 1/8 cable?

Some older Classe and Proceed amplifiers and the Theta Dreadnaught II need pulse signal to activate its remote trigger. You need a adapter to convert your preamp 12V constant triggering signal to pulse signal.

http://Theta Digital Dreadnaught II Owner's Manual

Remote Triggers
When the rear panel STANDBY TRIGGER jack receives a 5-12 VDC pulse, the Dreadnaught II will change its state from either standby to operate, or operate to standby, depending on what the current state is.
When the rear panel SURROUND TRIGGER jack receives a 5-12 VDC pulse, the Dreadnaught II’s surround buss will change from its current status, either on or off.
Use a 1/8” (3.5mm) mono plug for these jacks.

Before I had a Krell TAS amp and works fine also on the Mcintosh manual says the trigger 1/8 plug has to be stereo and on Theta says 1/8 mono plugs

Pull the trigger wire and see if that causes it to turn off.  If not, it is probably a bad relay.  An inexpensive fix.