Upgrade Lehmann Black Cube to Avid Pellar?

I hadn't realized that the there was a Black cube made that did not have the receptacle for the upgraded power supply integral to the unit.  I would like to help you further, but have no knowledge or listening experience with regard to the Avid Pellar.
You may possibly have more responses if you asked the AG community about the Avid Pellar (only). Instead of a comparison between it and the B/cube. I would say the probability of ownership of one phono pre or the other is far more likely to be found  then those that have owned both.

As far as the tube scene, the Black Cube is the only SS equipment I have. I may some day look into something like a Manley or ??, but for now I am pretty satisfied. I recently upgraded my turn table / analogue source with a new Lyra Delose and a Symposium platform, which resulted in even more detail and realistic tonal qualities from my Rega P5.

Thanks for the input. My cube doesn't have a wall wart but doesn't have the pwx either. I've thought about buying the pwx but would involve some electrical surgery because my cubes external ps is hard wired. Looking at an ee minimal now btw. Little more dough but I am eager to get into the tube scene w a phono pre

Just wanted to mentioned to you that when I purchased my Black Cube they offered the upgraded power supply as an option. Im not sure if you can still purchase the upgrade power supply separately since they  include it in the SE model, but if so , or if you can find one used, it may be something for you to consider.
Replacing the wall wart with the upgraded power supply was a significant improvement in sonics. It elevated my Rega P5 to the best sounding source in my system.