How to hook JL F112 sub to both preamp and pre/pro

JL Audio does not support that type of multiple connection, as you know, and I don't think you can get what you want with one sub. I think you probably need to get a sub crossover, Bryston 10b, or 2 way Marchand 44, and run your speakers full out. Set up the Pre/pro with no sub but full range speakers it will send all info including the LFE, through the HT theater bypass to the 2 channel preamp. When you output from your 2 channel preamp to the crossover, it will output all of the lowest frequencies to the sub from both LFE and music to the sub. So if your speakers go down to 40 hrz, you would probably crossover through the electronic crossover at about 45 - 50 hrz with a set slope, testing and listening for the best settings of course, until you get it perfect. All of the LFE effects will come through the sub without damaging the speakers.

No, you cannot, you absolutely can get competing signals that can damage the amp/crossover.

I have talked with JL Audio (and Velodyne, and B & W, and Sunfire....) about multiple inputs when I owned an F113. I bought a Rel B1 and sold the JL Audio so I could use one sub for both, with 2 sets of connection. Definitive Technology Subs do this as well.

I really think it's easier, and prob a better set up for a high end system, to buy an external cross over and keep your bad-ass sub like the JL Audio. You are integrating the 2 channel sub anyway, this just take it the next step and takes the lowest frequencies away. If you have speakers that truly go down below 30hrz, you may not want to do that, but then you can just let the full LFE signal go to those speakers anyway without damaging them, and you don't need any connection but the 2 channel, with the processor set to no sub.

The below is from the JL Audio F112/3 manual. As you can see, they specify NOT connecting 2 sources to one sub, as the tech at JL Audio told me previously. I don't know who Barry is, but I would check for clarification. I don't think you can do this with a JL Audio, I think it could fry the crossover and amp, unless they have changed the electronics significantly. Look again at the last sentence of the setup from the manual below.

SYSTEM CONNECTION DIAGRAM 1:
One Fathom to
Home Theater Receiver or
Home Theater Preamp/Processor
Most home theater receivers and
preamp/processors provide a single
(mono) subwoofer output. When
connecting a mono subwoofer output
to your Fathom, you will only use the
Fathom’s “Left or Mono” input.
Two connection types are available
for connecting the Fathom to your home
theater system: balanced (XLR or 1/4-
inch TRS connector) and unbalanced
(RCA-type connector). Balanced
connections provide superior noise
rejection and ensure proper grounding
between components. If your receiver or
processor has balanced outputs, we highly
recommend that you use them.
In the connection diagram at left ,
balanced connections are shown as solid
lines, unbalanced connections are shown
dotted. You will only use one of these
input connection methods (not both).

It will send signals at the same time, and I am 100% certain the manual is correct, and the tech, don't know the name, at JL Audio told me explictly that you CANNOT send both signals, you have to only have one going at a time, unlike Rel's or Def Technology.