You might also try America's Cardroom, Black Chip Poker, Tru Poker and/or Ya Poker. They are part of the WPN (Winning Poker Network if you didn't already know) and they accept bitcoin deposits and allow bitcoin withdrawals. There may be more skins on the WPN that accept/allow bitcoin deposits/withdrawals also. You can google the links yourself so you know I'm not secretly referring you. They accept U.S. players (I assume you are a U.S. player otherwise you'd be playing on Stars).
You can check
http://www.pokerscout.com/ and look for Winning Poker Network and get an idea of their numbers.
*NB: The only thing with these sites is that they
require WPN ID and address verification in order to withdraw even with bitcoin which may or may not be an issue for you.
*Addendum: Not sure what timezone you're in, but currently (10pm CST), Pokerscout is showing that 900+ cash game players are on the WPN right now. I'm sure a fair enough chunk of that are mid-stakes players.
Also just to add, I don't play on any of the sites above, though Betcoin.ag does have access to the WPN tournament pool.