I've never understood the issue people take with Iron Maiden's production. I think the record sounds great - raw, hungry, edgy. Killers on the other hand sounds way too polished for the early Maiden sound. It took a more melodic edge and someone like Bruce to make Maiden sound good with professional production.
The problem is that it sounds like a cheap demo with poorly EQ'd and recorded (extremely poor guitars. Way too much 2.5 khz.) guitars and vocals. High clearly audible noisefloor, clicks and pops, too little low mid on the vocals while too much 2 khz which resonates at certain notes causing my ears to go bananas - And it makes it sound a little telephone-ish. The spectral balance on the (especially the guitars) guitars and vocals, is completely unacceptable.
He went too far with the low cut filter and too easy on the 2 khz area.
The guitar sound isn't very good either .There is still room for more and they kinda need a little more low mid (and to balance, remove a slight low mid form the bass), WAYtoo much 2 khz, you can hear it ringing (which is the most sensitive frequency range of the ear and something you might want to tone down). Kill the 2,5 khz range and they would sound much better, and the vocals could benefit from having some 2.5 khz removed too.
The plate reverb sounds yucky to me (although that's what you had back in the day, if you didn't want to route the signal to a loudspeaker in the studio and mike the room) The rhytm parts aren't double tracked either and could at least have benefited from stereo miking. The drums are pretty good though, old school setup. It sounds nice as does the bass..
Killers on the other hand has an excellent production. I think you're wrong by saying that they needed Bruce to fit with great production. Killers is ace while Iron Maiden makes my ears beg for mercy.
If you guys have access to some audio processing program: Import a track, (Prowler's a good choice, especially as you hear a lone guitar in the intro) then apply an EQ (edit: preferably a parametric EQ) effect to the tracks insert or the master bus. Make the bandwith as narrow as you possibly can, the narrower the better. Then kill 2.4 khz and you'll find the album sounds much better. Take a second band and make a dip with broad bandwidth at 1,2 or so khz too and you have, within the context, a winner. The noise floor will inevitably get even higher though but whatever as well as too thick bass register for the era but that's an acceptable price.
Edit: The thing is, you won't know what you're missing until you've heard it with a good set of headphones with the EQ settings I typed. Makes it by far more listenable.