Not a good idea as one source will shunt the other, even if only one is active at a time.
No. Keep swapping the interconnects, don't try what you're proposing, it won't work well even if the unused component is off or unplugged from the wall. There was another thread about this general question not too long ago that went into some more detail on what Kal says, which you might find in the archives (although I don't know what you'd search under), but anyway that's the deal.
Continual swapping of your iterconnects will eventually lead to excessive wear and ultimatly the need for replacement (not cheap anymore). I too, recommend a well built "passive" line stage (unity gain, unbuffered and very high quality volume control). I utilize a McCormack TLC-1 Line stage (my 3rd one over the years). It has proven itself to be very transparent, without adding many of the influances of other (active) pre amps. There are others though, that can be had cheaper (than the TLC), and with less selection options. In fact, I spotted (what appeared to be) a well built little passive unit (home made) on the auction, not too many days ago. Passive Pre's (like this one) generally sell fairly cheap, and may suit your needs.
You could very easily pick up an excellent phono amp AND also a hi-end, passive pre amp for the returns on the sale of your Modulus 3A.
Some amps just simply " beg" for a passive pre amp (or, a the very least, direct input). These amplifiers may have inherant tonal qualities (that are so seductive) that any, and all, pe-amp influances will take away from their truth and their individual musical nature.
Try www.moon-audio.com and also parts-express. That sould get you started. There is also Parts Connection and of course, "Rat"-Shack (if you are desperate).
And also: .......... How could any one of us forget to mention the CREEK OBH (passive remote pre-amp) series. I beleive that Moon Audio still has them and Audio Advisor sold them (at one time). They are quick to sell on Audiogon. Perhaps what would help you the most.
I scanned through the Audiogon website looking for something for you. There was (very recently) someone that was selling high quality switches and pots (under tweaks or ss pre-amps) but I could not locate anything now. I did happen to spot a "Quasar" switch box that looked real interesting but it is FAR more than you would need. What caught my eye about it was the (apparent) high quality of exposed parts. Fairly cheap at about 200.00. I don't know a thing about it's insides (could be pure junk). It had a ba-zillion in's and out's and for both amps and source. Made me dizzy looking at it.