New headphone cable - Mogami W2799 or Canare L-4E5C quad

Either should sound fine under ideal circumstances. The 21AWG Canare will be a bit more robust under non-ideal circumstances. I prefer to use the Canare since it's fairly general purpose while still being small enough for whatever provided I buy "large opening" plugs (which cost the same anyway). Fewer random spools of cable translates to fewer dirty looks from the S.O., yeah? But the Mogami isn't a "wrong" choice if the thinner cable is important to you.

IMO those are going to be WAY too heavy for an IEM recable.  I wouldn't even consider it.  The shielding is overkill as it would only be covering a portion of the cable anyway.  I can't suggest another cable, but I'm holding 1 meter of L-4E5C in the air right now and it weighs at least 3 times what my stock RE-Zeros weigh.  I should mention that at least 60% of the RE-Zeros weight is in the actual driver units and plug.

If you're confident about the IEMs staying in your ears with that kind of added weight go for it, but I wouldn't recommend it. 

Eric

EDIT: didn't Canare come out with a smaller gauge cable recently or am I crazy?  To answer your Mogami vs. Canare question, I think Canare is more popular around here for whatever reason... I'm sure its not audible.

Wait, IEMs, Kibble's right. Sorry, I shouldn't answer too late at night, my mind skips over too many details. You don't want mic cable for that either - the extra conductor is fun for headphones, but extra bulk for IEMs.

A slightly roundabout trick which might be very effective would be to find a replacement IEM cable from whoever. Sennheiser IE8 cable is supposedly good about microphonics and so forth and may be worth considering. Your price per foot is likely to be poor compared to raw cable, but it should afford you at least one option from the options out there which is well-suited esp. in terms of avoiding microphonics.

From Canare, I haven't seen anything smaller, although they've had a normal / low profile pairing for ages. Doesn't mean it doesn't exist, but I haven't heard of it and it doesn't seem to be listed on their site.

Out of my own curiosity I did a quick search for IEM re-cables and didn't find specific cable models listed.  It seems most just buy new headphones, send them back to the manufacturer for repair, or canibalize the cable from another IEM (ie stock iPod buds).  Most IEM cables aren't made up of individually insulated wires like microphone cable.  Technically the wires are insulated, but it's a very thin layer of something (I forgot what) that you have to strip chemically or burn off with your iron so you can tin the wire.  It looks almost like they're anodized in different colors. 

Since you lost connection inside the driver units, you shouldn't hesitate to give the company a call and let them know.  I'm sure they'll be able to help you out.  If the connection was lost in the plug, that would be an easy re-termination with a nicer plug.