Re: Best stadium atmosphere?
Only spanish supports I've seen ever been rated above average have been Atletico Madrid & Sevilla. Think Gijon took 2500 to a big away game a couple of seasons back which made the headlines since as you said barely anyone in Spain travels in numbers. Think Madrid only take 500 to the Nou Camp and vice versa.
As for quietest support I've ever heard, Burton Albion. Went with a Plymouth mate last season in what was being tipped as their last game due to dire finances. Burton literally didn't make a sound all game and even their goal celebrations were awfully tame and quiet. Strange.