I had a full set of the Vixen LV's and they are ( were ) very good but I found that the Circle 'T' Orthoscopics I owned at the same time to be sharper on the moon and planets , both set were from the 3mm-24mm range .
Of course the 20mm eye relief was much easier than that of the Ortho's .
The LV's seemed to impart a slight dimming in comparisim , but it was invisible without direct side by side comparisim between these 2 sets .
I have a 2x , 2 inch Powermate and use it a lot in my Tak sky90 for the same reason Skysurfer uses his in his Genesis , with only 500mm fl to play with its hard to acheave higher powers with eyepieces alone so I use my Powermate often when wanting 200x plus in my Takahashi when using my smaller TV Radians , these I kept over both the LV's and circle 'T's , I find the Radian's have all the good quality's of both the LV's and Ortho's in one eyepiece , hence these are what I now own a full set 3mm-18mm .
The Powermate does not add anything to the view in my opinion except make it easier to use say my 3mm Radian and PM as a 6mm alone , nice bit of gear , but expensive .
This is my own opinion , oh yes I find the Radian's perfect in my 127mm f8 Istar achromat as well , these just work in that scope as well for some reason . Refractor's perhaps ?
Hope this helps ,
Brian.