I notice in Mica's picture that there is a silver colored nut at the bottom of the circuit board most likely holding the board to the case.  If there are any connections of this type that are used to make a ground connection to the case these are classic causes of the kind of intermittant problem you are having.  I would carefully tighten any nuts of this type if it looks like there is an electrical connection.  Again, be very careful not to touch the back of the circuit board or any components, even if the unit is unplugged, as capacitive discharge could damage you as well as the components.
- SoM

We don't do the grounding in the scheme described by SoM, but we've had a chance to talk and get you set up for the next test on your preamp.
 
Tonight, try setting the volume at no more than 1 and see if you get the popping and report to us how loud it is. Depending on what you find, you may want to change the tube, though it's rare that a tube would cause the trouble you're observing.  
 
If we aren't able to diagnose the problem remotely, then we will make arrangements with you to send in the preamp. In this case you'd email help (at) alembic (dot) com and get the process going.

I turned it down to between 0 and 1. The popping still occurred and was still very loud. If you believe replacing the tube should be tried next, can you give me a reliable source and brand of tube that should be used, and basic instructions for replacing it (I do have electronics experience). Thanks for your help.

If there was absolutely no difference in the popping sound then it's safe to say it's not the tube causing the problem. In this case, try replacing the 4558 opamp on the main preamp board.  
 
Let me know if you notice any change in popping sound after you replace the IC.
 
Also, just for good measure, please plug and unplug a connector in the effects return jack about 20 times. It's probably not the cause of the sound you're noticing, but you mentioned you had used the F-1X for years, and this part will corrode if it isn't excercised.  

I tried replacing the 4558, and also tried some contact cleaner on the jacks, without any success. Let me know if you more ideas or if I should send it to you for service. hanks.

Haven't gotten a response back. How would I go about sending this unit for service? Thanks.