A couple of things I want to add. First is that my understanding is that even though the TeleVue 2x Barlow looks just like other barlows - black top, silver barrel, black lens part on bottom - you CAN'T do what others have suggested with the TeleVue: you can't screw off the black lens part and screw that directly onto the bottom of your eyepiece to get 1.5x magnification. For once, cheapness has its virtures.

Second, I agree with Dr. Ski entirely. 15 and 25mm Plossls are very sharp, and have good eye relief. Unless you really dig those 68 degree views over the 52 degree ones provided by your Plossls - and plenty of people do! - there really isn't much of a need to upgrade your 15 and 25mm Plossls. Like he said, I'm sure the ES line is a fine set of EPs, but not 3-4 times as fine. As an example the other way, we all "know" that TeleVues are the best, right? I've also read that an ES gives you 95% of what a TeleVue gives you, but at 2/3 the price. Is that worth it to you?

As my signature shows, I have an f/12 5-inch Mak, broadly optically similar to your f/10 C6. If you think of the cone of light that gets delivered by our optics to our eyepieces, in our scopes, that light path, that light cone, is longer, more extended, than say the shorter focal length, focal path, of an f/5 or f/6 dob. Think of our light path as being more like those old-timey megaphones cheerleaders would or carnival barkers would use, versus something short and squat like a pyramid in a dob. With me so far? For whatever optical reason, EPs love the more parallel light beams that get delivered to them from long focal-length scopes like ours. It is just easier for EPs, and actually, even really cheap EPs to deal with that long light cone than a short one - there are less visual aberrations of all sorts, coma being first among them.

My Mak came with some Kellners, cheapo 3-element EPs, practically from a bygone era. I also have an 8mm TeleVue Plossl. And you know what? My 9mm Kellner keeps up with the vaunted TeleVue. Oh, don't get me wrong, the TeleVue still wins, but we're not talking tortoise and hare either. So the wonderful benefit of long focal ratio scopes like ours is we get to save scads of money by NOT having to buy the super-expensive EPs.

However, if you DO like those wide fields of view - and why not, right? - I can steer you toward the Agena Starguider/Astrotech Paradigm line. These are both the same EPs with different brand names silk-screened on them. I have the 15mm, and I recommend it. It is a nice EP, good eye relief, works well in both my f/12 Mak and my f/5 'frac, and the 60 degrees AFOV make viewing nice and comfy. If you feel put off by the fact that it's a "no-name" EP - you shouldn't, but in case you do anyway - then there's both the Celestron X-Cel LX line, and the Meade HD-60 line. Both of these lines also have 60 degrees of AFOV with good eye relief. I am a member of a different discussion forum as well as this one, and I have never heard a bad word said about either of these two lines - they are universally recommended.