Last I used Xoom, there were no pickup places in Palermo. They seem to cluster around Corrientes / Puerreydon. Not a huge deal to hop the subway, but sometimes places are out of money and send you on to another pickup location. Luckily there are several within a few blocks of there.
You should be aware that Xoom officially has some kind of annual limit on the amount you're supposed to bring in, which I think is something like US$4000, beyond which you're supposed to show some kind of government paperwork. Never hit that limit so I don't know. I've heard of a case of someone not being able to make a pickup because they were over the limit, but others I know haven't had that problem, and apparently it's up to the agent at the Xoom spot whether they want to enforce it or not.
Probably the best way to bring larger amounts of money in and get almost-blue-rate pesos, is using Bitcoin. If you're a US citizen, you can open an account with coinbase.com and link it to your bank account so you can buy the Bitcoins with a bank transfer (you don't want to be sitting on large amounts of Bitcoin because it's very volatile, so you do this at the last minute). Go on localbitcoins.com and look at some of the larger-volume traders here. Some of them work directly with cuevas downtown, so you transfer them the coins and go pick up your cash at the cueva (or you can meet them in person and they will hand you cash in crisp USD or ARS). Typically costs 4.5% these days, and you get blue rate so it's a lot better than Xoom.
