Mastering in Reason
1) What are you hoping to achieve from mastering?
2) Can this achieved in the mix itself?
There's certain things that can only really achieved from processing the 2-track output (such as certain dynamics processing and dithering) but all to often people start reaching for the eq, compressor and what not because "that's what mastering is about, right?". Sure, this is what a mastering engineer would do but that's because you can only do such broad strikes with a 2-track recording but if the ME had the mix in front of him/her then you can guarantee he/she would try and correct problems at the mx level because this is nearly always the most transparent way of problem solving. The old recording adage of "don't fix it in the mix" should be updated to "don't fix it in the mastering" for the electronic age.
I guess a better question to ask you is what are your intentions with mastering, i.e. what are you hoping to achieve?
Moreover, most people do some "pre-mastering" without even realizing it since its too easy to slap a multi-band compressor/dynamic equalizer/limiter on a lot of tracks today with the kind of processing power we have.
Having so many possibilities, i think its one of the cases where less equals more.
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I try to get my instrument sounds as close to the final mix as possible during the tracking/mixing stage.I think that's where your problem lies. When mixing you shouldn't aim to mix just close to the final mix, your aim is a final mix, i.e. when you're done there shouldn't be anything in your mix that needs fixing that you are able to hear with your equipment (and ears). If you think something needs fixing, fix it. Only when that's done should you call your mix 'final', and that's when you send it to a mastering engineer, who has much better equipment and tons more experience than you, to critically and objectively listen to it and make sure the song sounds well on all target devices, as well as fixing any flaws you didn't or couldn't pick up on during the mix.
I'm slightly in the same boat, however I'd be inclined to say the same thing as the other members here. When I use Reason, I have a mastering suite preset active from the beginning so I'm always mixing through that. I quite like the one called energy for the kind of thing I've been working on lately. There was a song I was working on, and I had been paying lots of attention to mixing as I went along, and when it came to 'the mastering stage', anything I did seemed to diminish the quality, except from setting up the stereo imager. Master eq was left flat too. Some people may see this as bad practice, but so long as you're applying eq(everything in it's own space) as you go, there shouldn't really be much left for you to do at the mastering stage depending on how scrutinous you are with mixing. If you're interested, this was the song I was working on https://soundcloud.com/echo-gecko
If this isn't acceptable sound quality, I guess I should shut up =p To be fair, it was an extremely rushed project. I'm normally the one asking for help, rather than giving it.