Is a Sub required for HT

My recommendation would be for a pair of smaller (10-12 inch) subs rather than a single large sub.  And use LF acoustic room correction if possible.  You can often find a used Velodyne SMS-1 listed online, if your AVR or processor doesn't provide it.

db

Yes, you want to hear s**t blow up. 

Also, if you want a super quality sub at a great price, look at my listing for a REL T9. The REL allows you to run a cable for the LFE sub on your Marantz and also to run the Speakon cable from your speaker terminals. The result is great HT effects and better integration when playing music. 

I have a SVS Sound Cylinder in my HT setup. I like it in that system for music and music channels. There are a lot of good options out there.

Regards