El.Acous. Low-E quieter than other strings

what type of pickup? if the most common under-saddle piezo, it means the bridge saddle is not well seated on the piezo element -- either because the bottom of the saddle is not flat or properly shaped to contact the piezo element intimately, or the ribbon piezo pup element has slid partially (or wholly) out from under the saddle in the area of the low E. you will have to remove the strings and saddle to examine this.

if a magnetic soundhole pickup you may need to adjust the polepiece under the E string or reposition the pickups.

if neither of those, tell us ...

Thanks for a quick reply :)
I'm not an expert in guitar repairs :) actually, I was always playing guitars as they are... but now as older I try to get what I think is right... and underpowered E string is certainly not right!
So.... I think (almost sure) it's piezo onder the bridge, but I'm not sure how would I remove saddle? (as I never did that)
What should I look for? I'm trying to visualise whole thing, but as novice guitar scraper, I'm not that confident :)

Are some photographs online available to educate me more on that subject?

Thanks
Vlad

remove the strings. pull the saddle. the bottom of the saddle should be flat -- easiest way to achieve this is 'rubbing' on a full sized file. I find a laminate ("Formica') countertop edging file works very well.

look into the bridge slot where the saddle normally rests. the piezo is usually a thin silvery ribbon lying in the bottom of the slot. some of these have six obvious "bumps" where the actual piezo transducers for each string are located. make sure that silvery strip covers the entire length of the slot, so it completely extends from one end of the saddle (low E) to the other (high e). if it does not, gently pull it back into place with a tweeters. if you can see the six piezo elements embedding in the strip, position the strip so each of these lies directly below a string. when you replace the saddle, tap it down into position so it fully rests upon the pickup. after restringing, the string to string volume may be a bit off. restore this by tapping on the offending string(s) directly over the saddle to help properly seat the saddle against the pickup.

Just one more thing - remember which way round the saddle was. Most saddles have the B string cut differently to the rest.