The first thing I would say, there is a lot of merit to what Greg said about keeping it stock. That is a vintage Ric and Ric's hold their resell value rather well, so any mods could devalue the bass to some degree. I also agree that nothing sounds quite like a Ric and I do very much appreciate the sound of a Ric. With that said, I did do the cap mod to my '79 4001 and I believe it sounded better for it. I just unsoldered the one leg of the cap, so it could always be put back to stock. Also, I have to admit that I have heard several Rics that have had Bart's installed in them, and they sounded awesome. The big question is what is it you are after, the best playing/sounding bass (to you), or is it more important to keep it totally stock?
With that said, I would highly reccomend making no mods that can not be reversed if you ever elect to. Do not make any mods that require you to alter the body or parts, such as the pickgaurd. If you want to add pickups that would require you to alter the pickguard, I would reccomend buying a replacement pickgaurd and alter that. That way you can keep the original in original condition. Also, keep all your old parts. You want to make sure that if in the future you ever want to restore it to it's original glory, you can.
Personally, the very first thing I would do (and you did ask for opinions) is have the the current problem with the bass fixed so that even if you do decide to mod it, you have all the parts, they are working, and you can restore it to original without any trouble. If it turns out to me a bad pot, although generally any old pot, and frankly, much cheaper pots will work just fine, replace it with a genuine Ric pot so as to keep it all original Ric.
If you decide to go with pups that require you to alter the pickguard, here is a link to a company that you can get a replacement pickgaurd from. I really do believe you would be better off altering a replacement.
Your Ric currently has 2) Volumes Pots, 2) Tone controls, one for each pup and a 3-position switch that is actually a pickup selector. Being a passive system to start with, I'm not quite sure how you would achieve that knob configuration that you were talking about. First, one of the pots would have to be replaced with a different kind of pot to work as the pup fader you suggested and the tone controls, I guess could be done with caps, but I don't think that would be very effective or achive the effect you are after. Also, by wiring it in this fashion, I believe you would lose the Rickosound ability, not that many people actually use it. Something you might consider is a passive tone control system. Here is a link to Rothstein Guitars who offers a passive tone control mod for bass in the way of a tone control pot. They also offers a 6 position varitone for the bass that can easily give several very different and distinct tones. These look very promising, work with any pups, and may give you that level of tone control that you are after.
welp, that was my 2 cents worth, hoped it helps and good luck with whatever you decide to actually do.
