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Do you have the ES 68, ES 82 or ES 100 (or combination thereof)?
You want to try and avoid true fields that are similar even though magnifications are different. e.g. ES 100-9 would have similar true field to ES 82-11. Of course for planets magnification is more important.
Anyway, there are many options. If you do not want to add more eyepieces and want either a Barlow or Powermate, I would still recommend the Powermate so there is no chance of vignetting.
My first choice would be to add more eyepieces to fill the gaps but I have never used ES eyepieces so cannot comment on any of them.
The 2x Powermate however I can comment on and say that it and the 4x are close to being optically invisible. My best 8.75mm eyepiece is my 35mm Panoptic and 4x Powermate but it does require a counterweight on my 10" Dob!
You do add 4 extra elements of glass to the optical train with a Powermate and if you have 6 elements already that is a lot of glass. This can be the difference between seeing a very faint star or galaxy at the limit of perception or not and I have actually noticed this difference but it is almost imperceptible.
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