Which subwoofer?

@bdp24 

Thanks for the perspective! I will do some digging at Audiocircle.

In my case my dipole (naked) 18" midbasses go from 65Hz to 275Hz. I agree there is some weight loss (vs a sealed 18") but I have not heard this level of articulation before. Large midbass, though!

I noticed you xo your GR Research subs at 180Hz. What's your view about how much difference it makes to have sealed vs OB subs below 70Hz? I'm sold on the advantages on dipole above that, but wonder if taking the sealed sub DBA below 65Hz wouldn't be the most practical approach and deliver the same sound quality.

@ewinskih01: Danny Richie uses a pair of OB subs in the front of his room and a pair of F12G's in the rear (in both his company's listening room and at hi-fi shows), the F12's in phase opposite that of the the OB's. I believe he uses 40Hz as the x/o frequency between OB and F12.

OB subs sound different from sealed and ported for a number of reasons. Not just because of their dipole SPL drop-off with listening distance characteristic, and not just because the woofers don't have an enclosure in which to produce resonances.

First, their dipole design produces a null on either side of the OB frame (for those who don't know, the waves from the front and rear wrap around the frame and meet in the middle of each side. Being in opposite phase, their combined output is zero---the two cancel each other). The result is that the sub---producing no output in the left-to-right plane, typically the room's width---does not energize the room's eigenmodes in that room dimension. Less "room boom"! 

Secondly, sealed and ported subs "pressurize" a room; the sub enclosure creates a separation between the interior of the enclosure and the air in the room in which it resides. When the woofer moves outward (in response to a positive signal), the air pressure in the room is increased (compressed); when the woofer moves inward (a negative signal, of course), the room's air pressure is deceased.

OB subs, in contrast, do NOT pressurize a room. The air on both the front and rear of the OB frame is vibrated by the woofer, but the room's air pressure is neither increased nor decreased. How could it be? The air pressure is moved around within the room, but that's all.

Some people like the sound/feeling of the pressurization of the room (Rythmik's Brian Ding, apparently). An OB in the front of the room with a sealed in the rear produces a compromise between the two---the lean/"taut" sound of the OB (optimal for blending with planar loudspeakers), the weight of the sealed. 

GRs OB servos are the cat meow. I use them about 60 hz down, and with MB columns phase plug tuned.
That is how I balance the weight of the bass. I like 80-280 or so..to be VERY fast.. I direct couple those drivers to BIG amps and use active XOs with full blown DSP multi xo points,  threshold, summation, on the fly correction and mic feed for OTF correction and testing..

Just so you know there is NO dampening when there is anything between the amp and the drivers.. That includes a passive XO.. NO DAMPENING.. No mater what you think.. ZERO.. My way it does.. just that simple. I'm not the only one either.. look around.. IF they will share..

I use to use VMPSs bass design, man were they easy for a passive design and DEEP bass. I still do when I need chest compressions..

They will do it..

GRs though.. just night and day how they work and pressure a room.
bdp24 couldn’t have said it better. When I coupled that system with MB columns.. I really got happy for the first time in a LONG time.. Man oh man does it work.. of course Planars and ribbons for the monitors sections.. again separate enclosures.. Lot of real estate though.. BUT you can actually get the sound perfect for your ears..

Where’s my transistor radio, go do a little sanding in the shop..
Clear coating plinths for the 124s.

Rel subs, in stereo, sized for your room.
Easy to adjust for various recordings and media, which is a requirement.
https://systems.audiogon.com/systems/8259The Time Machine doesn't lie.

Depends what your budget is suggest
Looking at Velodyne's new Deep Blue 15"
Or if budget allows DD15+