Squishy brass

I took a look at the "Brass" preset, and the attack on it is much too fast for sharp stabs. I think it's intended for slow swells. 

Freeze your horn track so you can see the waveform (bypass the compressor first). Zoom in and determine how long the initial attack time is for the samples you're using. If it's, say, 5ms, set your compressor's attack to 5ms and experiment from there, trying it 1-2ms less or 1-2ms more.

Yes, you want a fast attack for stabs and a slow attack for swells - in the synth/sampler's volume envelope, but not in the compressor's sidechain envelope. Two different things, with opposite effects.

Setting the compressor for a fast attack causes your sample envelope to have a slower attack, and setting the compressor for a slow attack preserves the fast attack of the sample's original envelope.

I can see how that might be a little confusing.


EDIT: even I was confused, reading my reply back! Let me take another crack at it...  A fast attack on the compressor means it kicks in right away, lowering the sample's volume very soon after the sound begins. This will clamp down the sample's level right at the start. A percussive sample will therefore lose some of the amplitude of its initial, er, attack. If you don't want to clamp that initial emphasis, you slow the compressor's attack time, telling it to wait a little while before enforcing the compression. You want to slow the compressor's attack time just enough to allow the sample's attack to sneak through before the compressor clamps down.

You know, now that I've had a minute to think about it, you probably shouldn't be using a compressor on a brass stab to begin with. It doesn't really serve any purpose unless you do want to soften the attack of the sample and make it a little less aggressive. But if a punchy stab is what you're after, you probably don't really want to compress it at all.