I am a firm believer, from first hand experience, in measuring your room and knowing what you are dealing with.

If you have flat bass down to the lower limits of your main speakers, you can run a sub without a crossover and just blend it in at the lowest octaves to fill things in. That is what I did with my Fathom in my old room.

If however, you do not have flat bass down to the limits of your mains as is the case with my system in my new room, you are likely to be able to achieve great bass by strategically locating a sub and using a crossover.

In either case, you could use a JL Audio Fathom or likely, an REL. I suspect any high end sub will have its own low pass filter and / or accept a control signal from a crossover. I love how my Fathom can be used and fine tuned in many different situations.

Measure your room and then let us know what kind of low end you have. From there, you will get more factual based input.

To reiterate, I believe that in order to get a smooth response you need to use a crossover. the odds of getting a smooth response without a crossover and measurements are very low. Even if you use REW or another measurement tool to get a smooth response without a crossover, it's very likely that at some frequencies the sub and your main speakers are cancelling each other thus putting higher demands on your speakers and amps and increasing distortion.

Need a CR-1?
 

I agree. I only use REW to show me the rooms issues, model my room to show possible placement locations and chart results. I do not play around with any of its EQ capabilities.

I believe most rooms have some issues with bass that a good crossover and sub will help alleviate through placement of the mains, the listening position and sub followed by educated setup of the crossover points, volumes and time/phase alignment.

My 2 channel sound presentation is most enjoyable with my passive crossovers in, many times I have removed my crossover but kept re-install. Pair of JL F110 really benefits with crossovers in. Gone thru active Pass Labs XVR1, Brystom 10B and Marchand XM126, settled with passive.
However, this is not to say all systems require crossovers, only your ears can tell you that.

Everyone should try both ways - with and without high-pass filter.
Every system and every room has its own tale.

I must say that it's (much) harder to setup/optimize subwoofers without high-pass filter, especially when low-pass filter frequency is not too low (and usually it shouldn't be too low) - the reasons are mentioned several times in this thread. But, in the end, it could worth the effort.

Since I'm not allowed to post links - just search for "Geddes Multiple Subwoofers".