First the usual bitching...
Without the proper monitoring conditions, anything you do means nothing. Trying to achieve the loudest master possible in shitty conditions just ends up in making compensations to make it sound good on your speakers, in your room. I should make this my disclaimer ...
Like said here, you work by ear and you can make things really, really loud untill it destroys the mix. How such destruction translates to you on deciding what loudness is good enough depends on the conditions of your listening/monitoring environment and the ability to make a proper judgement. You just have to understand when you crossed the line, regardless of what genre, but imo, thats easily recognized though lies in the very small margins.
I believe there's 2 sides to the loudness war.
1.)
Its okay to achieve a master that beats the loudness record as long as its not terrible to listen to. More than often these days its the way to meet the criteria you need to get recognition for your tracks.
This is purely a commercial perspective which has nothing on audiophile qualities one might rather hear when having a 50k hi-fi setup at home.
2.)
Its not okay, the reason people want things louder is because theyre to lazy to turn up the volume or because they have cheap amps in their system. They just dont make amps like they used to since its to expensive.
These days most amps are chip based and mostly that also implies that the headroom is far lower. Amps got smaller, cheaper and such amps are the most common type through which you hear music pretty much all day.
People that spend good money on their hi fi setup would mostly dislike any loudness because they hear the difference between a proper master song and one that is mastered purely for loudness.
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I'm all about detail, i'd like to say that id work the lot to entertain the audiophiles but i'd say that account to roughly 10% of the listeners. So the ethics on loudness have become irrelevant, loudness sells but still depends on genre though those genres are not subjected on this forum.
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@2Good; I think you're doing the right thing considering the setup you use. If you aim for your own mastering -0,1dB is the utmost but with subpar interfacing it can crap out on you but this can also happen with crappy plugins or gear (some of the best limiters only sound really well with just 2db reduction when they could do 6-7db).In the same way on the a subpar system i'd keep at -0.3dB aswell and just raise the volume on whatever im listening on untill i hear noise come up. I wont go for the loudest possible if the tools at hand wont let me or means i'd have to compromise to much.
In regards to using outboard gear and your i/o. Having to switch between +4dB/-10dB all depends on wether inputs/outputs are balanced or not.
In addition to this you have to check what the level are on in- and outputs. A class A brand piece of outboard could have a 26dBu output and the input it targets might only accept 22dBu.
Its all in the specs