As ClieOS has explained earlier adding balance control is not nearly as simple as you might think and the change in price is probably not going to be marginal(50 bucks) as you assume. I am not sure about this but if I had to guess the price jump would probably put such a product in a completely different price range, where it would be competing with a completely different line of products which would require even better set of parts, engineering and execution in order to be able to be competitive. And this completely defeats the purpose of the E12 which is now marketed as a good quality
budget portable amp which can run
power-hungry headphones.
So the question is not how many people are not buying E12 because it doesn't have balance control. The question is how many people are not buying it for lack of balance control versus how many wouldn't buy it if it had balance control and was priced at 200-250-300 dollars(or whatever price the new product would need to have to justify its production and engineering)?
The answer to this question, to me it seems is - you need a completely different product to satisfy those needs. I am curious if there is another product that would suit those 3 criteria I highlighted above plus being able to do balance adjustments? In other words is there a good quality portable amp under 200 bucks that provides balance control and can power power-hungry headphones? If not, at what price range is the next viable solution. I haven't needed balance control, so I have not researched the marked about this(but on the fly I can't think of a single one that supports balance control). If you find a solution, I'd be curious to hear what price-tag it comes with. Good luck with your search