Subwoofer speed is in the room, not the box

@clio09 Again, Roger is correct. Power and damping drive subs the best. But the real difficult aspect of ESLs is the very low impedance way up high. My tube amps have an output impedance of 1.75 ohms, but my speakers drop down to 1 ohm at 20 kHz. If you look at the system's amplitude curve, it starts rolling off at 12 kHz  and by 20 kHz it is down 60 dB! I just got my DEQX Pre 8 digital preamp and the plan is to bi amp the transformers. I am going to use a Bricasti Design M25 to drive the high frequency transformer. The Bricasti has a very low output impedance and will easily take the speakers up to 20 kHz. It also has a huge power supply and will continue doubling down to 2 ohms. Sonically it reminds me of my old Krell KMA 100s. I ran those till they self destructed. Roger West recommended a crossover frequency of 5 kHz, so I will start there. In an interesting inversion the tube amp does a stunning job of driving the lower frequencies and would happily take the speakers down to 24 Hz, If I let it. Solid state amps suffer because of the high impedance, the MA 2s do not. 

@gdaddy1 That is true, the first motive of any company is to stay alive and to do that they have to sell product. The problem with REL is it seems to be the only motive. Anyone can shove drivers into a box and make up a marketing story. If you want really bad bass and a lot of distortion follow Rel's instructions. I have been using subwoofers since 1978 with electrostatic speakers and have been through every possible permutation. Commercial subwoofers are so bad I design and build my own. Randy Hooker made the best subs you could buy, brilliant design, but way too big for modern sensibilities. Mine are the best subwoofers you can't buy, but unlike Hooker's subs you need a lot of power to drive them. I took me 30 years and 3 trials to get them right. You can see pictures of them in unfinished form at the link below. They are finished in high gloss black polyester and clear coated. If you have questions about the design fire away. There will be a full pictorial of their construction in case anyone wants to have a go at it. 

@mijostyn 

Your subs look great. Are they push/pull or dual opposed?  Have you ever tried precast concrete tubes?

FYI..I don't use REL subs. I just use that crossover/volume method. My experience has yielded a great end result that's cheaper and easier and sounds great to me.

An advantage of going to an audio show is you get to hear some of the best speakers in the world and then go home and compare. Otherwise the imagination can take take over in beliefs that are not based on reality. IMHO.

@clio09  If the amplifier could talk and express an opinion on filters, slopes, and frequencies, not to mention phase and damping we might be a bit surprised. After all its the amp that is coupled to the speaker.

During the development of their sensor technology beginning with the SMS-1, Digital Drive and finally with their DD Plus, Velodyne Acoustics (USA). While it doesn't talk it simply listens to the mains, subwoofers within the room as it makes your suggested adjustments and many more.

Briefly, using a Texas Instruments platform and Code Composer Studio which enabled a broad range of coded controllers of multiple variable parameters within multiple low frequency bands. The end result are audibly and graphically viewable automatic and manual parameter adjustments saved to memory presets. 

With the Plus release the manufacturer fully adopted the subwoofer crawl as the first step implementation. 

If you put the mains through a capacitor that was not previously there, what does that do to the overall phasing of the drivers, including the mid and high that now have an additional capacitor in their circuit?

What influence would this have on soundstage and imaging?

I run my speakers at 2.5 ways using an 18", a 9.5", and a Heil. I’ve put the cap on the 9.5" (run as three ways) and it is a loosing proposition as I lose a lot of the low end volume near the crossover frequency. I’d never considered it with respect to soundstage and imaging.

Also, different types and makes of caps have slightly different sounds. It seems that putting a cap on the mains will then create some degree of coloration that would not otherwise be there.

In another vein, yes a small sub is "faster" than a larger sub at the same volume level.

Ultimately the volume is directly related to the volume of air moved. Because a smaller cone must move further to displace the same volume of air as the larger cone, and it must make its displacement in the same amount of time (Hz), it travels a further distance in the same amount of time. Speed is defined as distance over time and a faster speed means that a further distance is moved over the same time frame.

So, then, the smaller cone is ligher making it easier to move, possibly reducing "flop/flutter".

But again, the cone travels faster and force is defined as mass times velocity squared. So while mass is a linear function, speed has more impact on the force required to make the cone change direction because it is based on the square of the speed and these forces must be overcome at each cycle of the cone.