U2 sound quality has always been a little deficient. A deliberate "edge" sound to their music and poorly defined bass makes a great sound in a pub or in the car but less so on an audiophile system. Most of it has a real "garage" rough sound to it - I believe this is all intentional. Some albums are better than other - the long time Daniel Lanois collaboration is mainly responsible for this...Daniel is well respected for an amospheric or gritty sound ( rather than polished ) - he does Bob Dylan stuff too.
There is very little you can do, like the Stones - there is a roughness in the sound that U2 puts out (although the Stones did have an interlude in well recorded sound with Emotional Rescue and Tattoo You) I think Rattle & Hum is U2 at their peak.
If you prefer great sound in rock/pop then you must choose other artists (who deeply care about sound quality) and their sound engineers....Mark Knopfler/Chuck Ainlay for example. Tom Petty ( a sound fanatic). INXS (crazy about sound quality). Trevor Horn is probably the most famous engineer for his polished sound (remember Frankie Goes to Hollywood, Duran Duran Rio, Buggles Video Killed the radio Star, Grace Jones ,Seal, Simple Minds etc...all that highly polished stuff from the 80's). Alan Parsons is another famous engineer ( Pink Floyd ). An then there is Steely Dan....
Not to belabor the point, but you get the idea....IMHO Bono doesn't like being upstaged by anything or anyone ( including his backing band ) ...this may explain the U2 sound quality - nothing distracts from Bono!!!
