dont forget that going outta the mpc ,digital-to-analog conversion, into a semi-pro (noisy) compressor like the alesis with a shyte signal-to-noise ratio, then back into the mpc analog-to-digital conversion again, is gonna add a LOT of artifacts: all that conversion distortion from your two(!) 16bit a-to-d conversions, the noisefloor from the analog compressor, the distortion from the low slew-rate of the shyte op-amps used in that analog compressor's audio circuitry, and the rf noise you picked up along the way using those cheap radio-shack cables

.....
unless you got a nice 24-bit a-to-d for the trip back, and a real nice analog compressor ...i'd recommend to stay digital once you're in the digital domain, there's plenty of great plugin available to manipulate your sound and avoid the added artifacts of un-needed conversions..
if you really wanna compress analog with semi-pro gear, i'd recommend using compression BEFORE the first conversion , ie, when sampling from vinyl/drum machine/keybaoard/mic...
i had great results sampling stuff i wanted to crush, with akai's hexacomp, a cheap but sonically powerful multiband compressor in a pedal...