I almost commented on this in my original post (but I was too lazy). I think that after you negative double, the burden is on your partner to act over 3♦ (after all it is all white, MPs, and he has a singleton). It's possible others will disagree with me. If he passes, your hand's action is also not clearcut (although I suspect I would pass in tempo). The auction you originally gave and the auction after a negX are a lot different because a. in the first auction partner's 2♠ bid was free, after a neg X he has fewer options b. in the first auction you had not described your hand at all / your hand was great in context, in the second auction you already described the key features of your hand