At wits end - constant howling/fighting

I've had my two neutered boys for 12 years now. I love them dearly, but the last 6 years have been torture. They constantly howl/cry/meow. For no reason. They have toys, occasional cat nip, clean litter boxes, fresh food and water. Sometimes attention is what they need and solves the problem for 4 minutes, but they never want much attention and often run away to howl more.

Took them to the vet and they don't demonstrate that behavior when they're in the office, so the vet has only had basic advice for us. But. -- I do know they don't have any health problems and they're up to date on vaccines.

They're declawed, otherwise I would have honestly tried transitioning them to being part-time outdoor.

I feel like we have tried everything. We buy expensive real food for them, fresh water, clean litter boxes, behavior training with treats, toys, ignoring them when they cry, spraying then with water when they cry, separating them when they fight, the calming diffusers, the calming collars, calming spray....

It's driving me insane. I'm ready to give them away. Which breaks my heart. I know they must be stressed by how frustrated I am and how crazy they make me.

Please help. Chances are I've tried your suggestions after searching google for many hours the last few years, but I'd welcome them anyway. Hoping there is something out here that works before I lose it.

Please don't spray them with water

We no longer use the spray bottle, which was a mist spay (and recommended by our vet) to break up fights.

Just realized I had posted "when they cry" instead of "when they fight"...sorry for the confusion. Water when they fight, ignoring when they cry. But we are honestly not doing anything right now because nothing we tried worked. For over six years.

Are brothers? From the same litter?

Spraying them probably made it worse as they might have link the spraying to each other

The spraying was maybe practiced for...two months? About 3 years ago. It doesn't seem to be any worse. I've just...I've just had it.

They are not from the same litter, but they were born on the same farm and raised together. I had the slightly older one for about 6 months before getting the younger one. I call them "brothers" but they're probably genetically not related.

The most effectively broke up fights but they still fight as often.

Mist* not most.

If you havent tried this yet I would separate them for at least two weeks and reintroduce them following the article below

http://www.thecatsite.com/a/the-ultimate-yet-simplified-guide-to-introducing-cats

I think that might stress them even more. If one is at the vet alone, the other one looks for them and cries harder than their average constant sad howling and then they both end up bathing each otjer and being quiet. Momentarily. Do you think that quiet time would help and be extended if they miss each other for that long? Or will they just both be miserable for two weeks and then fight to re-establish dominance and go back to being little terrorists?

The fighting isn't the major issue, though it is so frequent that when added to the major issue (the near-constant screaming and crying in corners of the house) it just adds up to misery.