Is anyone using a TRS 1/4 from their bass directly into the F-2B?  I assume by looking at the inputs that a splitter cable is needed to preserve the stereo signal.  Is that correct?

Yes; if you have a non-Series Alembic with Anniversary electronics, or if you are using the 1/4 jack in stereo mode on a Series instrument, then you can use a TRS splitter cable to send the neck pickup into one channel of an F-2B and the bridge pickup into the second channel.  Instructions for how the cable should be wired can be found here.

Perfect.  Yeah - I have anniversary electronics and will be outputting in stereo mode.  Thanks!

Does any cable manufacturer make an off the shelf TRS 1/4 90-degree instrument cable?  The splitters are pretty easy to come by as well as straight TRS cables.  The angled ones are eluding me.

Here you go friend !
 
Available here ; TRS& TS
 
Happy cable making,
 
 Using heat shrink tubing helps ensure a firm strain relief  as well !
 
http://www.markertek.com/Connectors-Adapters/Audio-Connectors/1-4in-Phone-Connectors/Switchcraft-Corporation/SW226.xhtml?utm_medium=shoppingengine&utm_source=googlebase&cvsfa=3786&cvsfe=2&cvsfhu=5357323336&gclid=CKT9m-Xi_roCFWEaOgodQ1oAGg

Jerry,  I have a store bought Whirlwind with a 90 TRS and I also have a Custom made Bayou Audio cable that was not too expensive if you are not that handy with a soldering gun.  The Whirlwind I have had for about 34 years.

I placed an order with Bayou.  Thanks for the tip!