All:
Last fall I purchased a F-2B (serial # 544) and am just now getting around to trying it out.
 
Caps look good, foam should probably be replaced.
 
I connected it to by black box and my new (to me) DS-5R.  The signal is very weak, and when I turn everything all the way up, distorted.
 
I replaced the tubes (Sylvanias) with Groove Tubes Silver Series.  Same result.
 
What should I try next?

Is it distorted in both channels the same?

Yes. Both channels are behaving the same.

Make sure you have exercised the Mixed jack (M) on the back. Plug and unplug like 30-50 times. If that doesn't work, it might need replacing. There is also a more detailed cleaning procedure in the FAQ

I will try that as soon as I get home.

Success!
 
Kind of.  The issue may have been a case of pilot error, I have my gear in a server rack and use a patch bay, and I may have routed the output of the M jack to the bottom row of the patch bay when I wanted the top.
 
But now I may have another issue.
 
I'm running the DS-5R in stereo.  Using the front outputs to drive the F-2B (Bass to A, Treble to B).
 
Pickup selector is on the bridge pickup.  My expectation is that only channel B will be driven, but both channels are.  I get the same behavior if the pickup selector is set to the neck pickup.
 
Note that I believe the signal coming out of the bass is fine, because with the pickup selector on the bridge pickup, the neck pickup volume as no effect, and visa-versa.
 
I think I'll go read the FAQ on the F-2B.

That seems odd for sure. When I have the DS-5 switched to stereo I get each pickup out of the respective bass and treble sockets. When it is switched to mono I get both pickups from the bass socket and the bridge pickup from the treble socket.

Try exercising the DS-5 Stereo/mono switch if it hasn't been used in a while.  
 
Keith

Excellent ideas.
 
The plan for tonight:
1. DS-5R in mono mode, see if I get both pickups out of bass, bridge pickup out of treble.
2. Excercise the DS-5R stereo/mono switch.
3. Use a non-Alembic bass.  Plug into input A, see if the signal also ends up in B (and visa-versa)

The two channels of the F-2B should definitely be completely separate. I have used it in the studio with bass in one channel, bass synth pedals in the other, playing simultaneously, with no leakage.

Maybe it?s the pickup selector switch on the bass.
 
Using a different bass, I verified that there is no crosstalk between channels on the F-2B.
 
Using the Series I, DS-5R in stereo mode:
 
Pickup selector set to both pickups:
Neck pickup drives bass output, bridge pickup drives treble output.
 
Pickup selector set on bridge pickup:
Bridge pickup drives bass and treble outputs, neck pickup has no effect.
 
Pickup selector set on neck pickup:
Neck pickup drives bass and treble outputs, bridge pickup has no effect.
 
W E I R D.