Hey Adiabatic, I've got some suggestions that might help with people's complaints about your images being dark.
First of all, try viewing your images on a different screen than the one you work on. You might've done this already, but if you haven't definitely try it. I strongly suspect you have the brightness turned way up on your main monitor (and it's also possibly in a fairly dark room) so viewing the images on a different screen (like your smartphone), in a different environment (outdoors or in a brightly lit area) might give you a better sense of what we're seeing.
Secondly, I think the biggest issue isn't so much the darkness of the images as the lack of contrast. Your characters blend into the background, their features aren't always visible and, often, figuring out what's happening as a reader can be difficult. Art students are taught to squint occasionally when drawing a subject because it helps isolate the important information (like silhouette) without getting distracted by details. Squinting is also a quick way to find the areas of an image with the highest contrast. As an example:
Attachment 94803
(click for large size)
Our eyes are drawn to high contrast areas. In the original image, these are the speech bubbles and the throw pillows, particularly the one on the left (since it also happens to be right beneath the brightest caption). That's unfortunate because the left side is possibly the least important part of the picture.
My edit on the bottom is crude and lacks a lot of the texture of the original, but the important information is easy to read even as a thumbnail. We're looking at a woman lying back. Additionally, high contrast gets the message across while retaining the sense of her environment being dark.
Google some examples of "chiaroscuro" or "tenebrism" and you'll see how artists have used contrast to create images that are dark, but still very readable. I also came across this YouTube video titled "4 ways to light a scene so it feels dark (but actually isn't)". It's addressed to filmmakers but the same principles also apply to comics:
http://nofilmschool.com/2016/10/4-wa...-actually-isnt
I hope I haven't offended you and don't come across as patronizing. It's obvious you put a lot of care and effort into your work and it's a shame for it to get lost because of something technical. Hope you find this helpful!