In regards to XLR attenuators, I use the ones from Naiant. Happy to support our team.
If you are going into an M10, that's a 1/8" stereo miniplug, right? I would go to Walmart or Radio shack and get a simple 1/8" in-line volume control for iPod ear buds. It's cheap, like $10. Are they likely to be top quality? No, but it's probably good enough, especially if you are dealing with a hot signal. Use your RCA's > 1/8" cable, and then put the headphone volume control in the line between there and the M10. Start off with it turned all the way up (no attenuation) at which point it's just a straight through wire. If you have to turn it down a little, you just turn it down.
If you cringe at the idea of using a cheap thing from walmart, I'm sure there is someone glad to sell you an "audiophile quality" one which is basically the same thing at a higher price.