Don't do this at production stage!

Think of all the music you run through that amp - professionally produced stuff, made in staggeringly expensive studios, mastered by the best ears in the business. How is it that even their stuff sounds better with loudness?

Loudness is a function on your amp that makes things sound better on hifi speakers by boosting the bass and the high end. On hifi speakers, this can sound quite nice. Same goes for low to mid range headphones. But what you want for production is flat flat flat. If you bought expensive speakers, you wouldn't need loudness - you'd be able to hear your mix clearly and appreciate the relationship between all the parts - how cleanly the bass sits, how heavy the kick thumps, how sparkly the hihats are etc. If you just whack more bass and more treble on everythign, when it comes to playing it through your amp, if you put the loudness on it just degrades into noise.

When you're producing, you need to produce music that sounds good everywhere - on a rig, in a club, on a stereo, on your ipod etc. You can add bass to taste on each of these devices, but if you but extra bass in at the production stage, you'll drown people in bass they can't remove when they listen to it on different devices.

Another note, adding extra bass gobbles up all your headroom. Adding extra bass prevents you from making the track louder at mixdown and mastering stage. A good mastering engineer will usually trim off bass from your mix, rather than add it on. As you turn up the volume on a well balanced, mastered piece of music you'll increasingly feel the bass. If its got too much bass it just turns to mush the louder it goes.

ramble ramble. Anyway - long and short - DONT DO IT!
Go for flat, balanced stuff that sounds good on all sorts of systems. The loudness button might make things sound nicer, but it wouldn't do your productions any favours