I'm looking at the Radial JDX 48 and I'm confused why it features reactive load circuitry when it is designed to be inserted between the amp head and cabinet.  Why is a reactive load necessary when the speaker cab is in the circuit?  
 
Edit: For some clarity....as I understand it: 
 
On the input side...The signal being tapped off of the amp head is a result of the relationship with the speaker cabinet OR a reactive load that is emulating a speaker cabinet.  Either way, the signal coming out of the back of the amplifier is effected by what it is loaded with. 
 
On the output side...the signal leaving the box into the interface would sound like crap without speaker emulation.  At this point in the signal processing, we have a good "authentic" signal because of the input side above, but this signal is still designed to hit a speaker cabinet - so we need speaker emulation circuitry to emulate a cabinet being mic'd.  
 
 
So, speaker emulation and reactive loading are two different things.  My question above is concerning reactive loading on the input side. Why would I need a reactive load for the input side since the speaker cabinet is already doing that job?  
 
 
 

because the line out from the JDX is going directly to the interface.....
 
it needs some kind of load applied directly to the signal it is tapping from the amp,
or it will sound of sh!te.
 
 
i use a Palmer PDI-09, the same way as you intend to use the jdx.
 
it's not a reactive load per say, more like a fixed cabinet simulation using a hardwire circuit to fine tune the signal to sound similar to a 57 on a 4x12, 1x12 open back or 1x12 closed back.
 
something like a Suhr reactive load box is an actual reactive load.
 
my Weber Mass Lite, that i use in conjunction with the palmer, is a REACTIVE load box, that also uses some resistive load.
 

batsbrew
because the line out from the JDX is going directly to the interface.....
 
it needs some kind of load applied directly to the signal it is tapping from the amp,
or it will sound of sh!te.
 
 

 
But isn't that what the speaker cabinet is doing since it is still in-line with the amp? See, I get that if I wasn't using a speaker cabinet you'd need some kind of load for the tap off of the amp head - but if the amp head is still "seeing" the cabinet, then doesn't that mean the cabinet is loading the amp?  
 
I'm in education mode right now...big purchase decision coming up...

i guess the line out happens before the send to the cab.
 
the cab still has some kind of load on the head, 
but the output of the jdx going to the cab, is kinda after the fact.
 
all i know is, the palmer sounds damn good.
so will the jdx.
 
 
you cannot use those line outs to do speaker impulse reponses, tho.
 
they are not for that.

just for fun:
 
https://youtu.be/_4f-RhcJaEo
 
a quick thing i did to just test my vid-to-audio line-up...
 
two channels, mono, hard panned left-right...
 
one is a miced open back 1x12 with a Heritage G12h30-55hz.....
the other is the palmer.
 
if you listen in the phones, and pan signal or mute the sides, you can compare

 Edit, The diagram I found was for the wrong unit, I see you figured it out anyways and this box does always use the speaker. Basicly a replacement for a mike on a live stage or studio rig. I thought it was one you could use to replace a speaker. 

Ok, figured it out.  I guess I should have just read the online manual to begin with.  Apparently, the "reactive load" they are talking about really is on the input, and uses the unique relationship between your amp and cabinet.  I was afraid it was ignoring the cabinet and using its own reactive effects. 
 
When the term "reactive load" is used, I immediately think of reactive load boxes which *replace* the speaker cabinet. This is a bit different, but cool nonetheless.    
 

Amplifier Input and Speaker Thru-put: The FROM AMP input and TO SPEAKER thru-put jacks are paralleled together and provides the means to patch the JDX 48 in-between the amp output and loudspeaker.
 
Transformer: The transformer is the front end of the JDX 48 circuit and is used as a reactive load that responds to the various electrical phenomena that dictate how a particular amplifier and loudspeaker sound together. This is what makes the JDX 48 unique when compared to the “load box” approach. A load box simply burns off the excess energy into heat and completely ignores the amp and loudspeaker interaction. By capturing the reactive effect between the amp and loudspeaker the JDX 48 comes closer to the original tone than any other interface system.
 
The transformer also pulls double duty by electrically isolating the signal passing through it, effectively blocking noise caused by ground loops. This is important because the JDX 48 connects a guitar amplifier to a recording or PA system that may be powered from a different AC main circuit. Without the transformer a ground loop may form that can cause buzz and hum.