Thinking I need a Sub...

Yes! If you are able, get two. Although I cannot comment on specific REL subs, I think all small monitors require subs.

My other option is to go up to a tower like the Nola Contender...I think I'd rather do this but worried it still may not be enough?

if you can swing a Nola Contender I would do that.

I'm working on some Contenders...:)

There's about a $1900 difference between a pair of Boxers and Contenders. You will not get the bass extension or room pressurization from the Contenders that you'd get from the right powered sub(s) for less than that.

Since you're hearing your Boxer woofers distorting, you'd benefit from a sub that can send a high-pass signal to the Boxers and unburden those little woofers from attempting deep bass excursions.

One subwoofer product line that comes to mind is the new JL Audio E-series subs. They start at $1500 MSRP for the E110, which is only down 3dB at 23 Hz. According to the Stereophile test measurements, the Contender is down 3dB at 50 Hz.

The cool thing for your situation is that the E-series panel can accept low level input from your preamp, run it through a 4th order crossover, and send the high pass signal back to the amp that powers your Boxers. This will make a significant improvement in the Boxers' clarity and dynamic range.

I can't imagine getting anywhere near this performance from the Contender. You may need to take 2-4 hours integrating the sub/sats, but it will be worth it. Not only will the bass be stronger, more dynamic, and more extended than what the Contender can offer, you won't be adding cabinet noise to your mains, which is just as significant for low level detail. I've yet to hear a floorstanding version of a monitor (except Wilson, Magico, & co.) that is as clean and resonance-free as the smaller, simpler, and more rigid 2-way monitor little brother.