Is a Sub required for HT

Yes.

No....but!

Your receiver's bass management will route all the bass to existing speakers.  The experience of watching a movie with a well i tegrated sub is transformative.  Especially naval movies, but integrating a sub well is a ton of work to do rignt.  if you cant take that effort, leave it out.

To do subs well usually requires bass traps and DSP in that order.

I already have GIK room treatment, including bass traps. I don't understand why you say integration for HT is difficult? I won't be using a sub in my 2-channel system.

I don't understand why you say integration for HT is difficult? I won't be using a sub in my 2-channel system.

It's really not hard to add a sub to home theater.  2 channel yes, but HT is a walk in the park.

I agree that the sub will make a movie that has that LFE channel a whole different experience.

Looking at the manual for your Marantz receiver, the default setting for Subwoofer is "yes". If this has not been changed, you are losing the LFE channel. Make sure it is set to "no", then it should send the LFE signal to the main speakers. This can make a difference in overall performance. Take a look at page 196 in your manual.