XLR to RCA plugs or cable

One thing to keep in mind is that RCA inputs prefer to see an unbalanced consumer -10 dB level. Most balanced XLR levels are at bare minimum +4 dB output. Without stepping down that level, it’s possible you could overload the RCA input, not to mention creating some nasty ground loops, by using a simple XLR to RCA converter cable. The more correct way to do this is with a passive step down isolating transformer device; like the Jensen PC-2XR.

PC-2XR

The Ebtech Line Level Shifter accomplishes the same task for a cheaper price (and still has a ruler flat frequency response). With the Ebtech however, you have to use a 1/4" TS (tip/sleeve) cable to RCA, instead of an RCA to RCA like on the Jensen model.

Ebtech LLS-2-XLR Line Level Shifter

Are there no RCA output jacks on the phono stage? If there are, exactly what are you trying to accomplish here by using the XLR outputs?

@dlcockrum 

Turns out the Cardas adapters are wired internally to join Pin 2 and Pin 3 together. 

Most solid state opamps do not want to be driven in that fashion, and will probably overheat and fail over time. I'm not sure how comfortable a tube's + & - output would be, if driven in this fashion, but I'd personally never want to risk it myself. 

One thing to keep in mind is that RCA inputs prefer to see an unbalanced consumer -10 dB level. Most balanced XLR levels are at bare minimum +4 dB output. Without stepping down that level, it’s possible you could overload the RCA input, not to mention creating some nasty ground loops, by using a simple XLR to RCA converter cable.

@dpop I don’t think a phono cartridge to phono stage is close to -10dB.

I don’t think a phono cartridge to phono stage is close to -10dB.

@holmz @lohanimal There was no mention of gain of the phono preamp (input vs output). If the phono preamp has a RCA line output, then that output is generally speaking rated at -10 dBV (just like any other RCA output). If it has a balanced XLR output, then that output generally speaking is probably rated at +4 dBV (just like any other balanced XLR output). That’s the general consensus of RCA vs XLR outputs. Now of course the voltage of those two outputs will vary, but if we're going to categorize RCA and XLR outputs, that's how they're categorized (-10 dBV & +4 dBV respectively).

Differences Between Mic Level and Line Level

You’re right @dpop - I has read it as into the phono stage, rather than out of it.
(Sorry Mate.)

That Jensen isolator is making a lot more sense now in that context, but also I would try a Neutrik first, and then the OP already has the XLR cable in case they need the Jensen.