Jas, it's easy to get confused with a question like this, because everyone has different experiences and opinions they offer. All the top end eyepieces are very good and you won't make a bad decision with any of them. But if you have decided on TeleVue's then stick to your guns and buy them, they are the best IMO.
Powermates are a newer, better version of the Barlow. They have better coatings and don't increase the eye relief of the eye piece. A 2x in your 12" will be useful, but I hardly use mine because I like to use eyepieces straight up. I wouldn't suggest spending your money on 2 powermates or barlows, I don't think you would use them both enough to warrant the cost. Put the money toward another eyepiece.
Naglers are very nice, but for a few extra dollars you can get the Delos. Apart from the slightly smaller field of view, the Delos is a better eyepiece in every way.
On the other hand the perfect eyepiece for an un-driven Dob is the Ethos. Al Nagler's suggestion for the perfect eyepiece set is the 21, 13, 8 and 6 Ethos. I was going down that path until I bought a Delos to try and liked it so much I stuck with them for the shorter focal lengths.
IMO, if you are planning on spending Nagler type dollars, spend a little bit more and buy a set of Delos. Just another opinion to confuse you more.
