Try this signal path and change your patch cables to interconnect cables (as soon as you can) instead of instrument cables:

Bass>Preamp>RNC>Amp

Also, if you're using this in front of your amp you're going to have to reduce Gain on the RNC because the amp is not built to handle high gain levels at the input. Hi impedence/Low voltage. So get good DI signal levels going from your preamp to the RNC then reduce Gain on the RNC until you get a level that doesn't clip your amps input .

Might be an overly fast attack time as well.

thanks for the reply. its not a gain problem cuz as the threshold is dialed down so goes the distortion and as soon as no compression is being applied so goes the problem. more comp = more dist.
also i need to connect the bass first in this application because i'm concentrating right now on contolling the input levels going to the tube pre for more precise control of the tube distortion from the ampeg.
unless 1/4" mono cords cannot be used with this epuipment i think its the units fault - but i've been more wrong than this before!

ok, so it was in fact the attack time.
thanks david.
could i ask for a quick explanation of the way this is a problem. my other compressors don't do this so i assume its the digital part of the rnc?
if i want to control the most immediate part of the sound (attack of bass guitar note) shouldn't (or at couldn't) the attack of the rnc be as short as possible?

When the attack time is too fast, the compressor starts acting on individual waveforms and changing their shape, thus generating other frequencies. Keep the attack greater than about 25 ms (3:00 on the RNC dial) and you should be fine.

All compressors should to this. Also, the RNC is digitaly controlled, but the signal path is all analog.

You also want to be sure not to use too fast a release time! A slower release time will sound more natural and keep the bass sounding more subdued. A fast release time will increase the " apparent loudness" of the bass. The same holds true about what David said regarding the attack time, the same can be true for the release time. The bass will become more aggressive sounding when the release time is speeded up but it too will cause a distortion problem. So the bottom line is, LESS IS MORE and slower is better for a cleaner more natural sound.

Slower at 50
Ms. Remy Ann David

learned and done. sounds great. thankyou - matt

David, that was very cool troubleshooting. You taught me something new too, thanks.

Glad to be of help! 8)

While the answers given in this thread are right on the money, I thought it might be useful to see the "official" answer from the FMR website:

Why does the RNC distort my bass guitar?
At the risk of sounding too esoteric and philosophical, there are many universal laws that are, many times, inconvenient (like, say, gravity). Well, there's a mutual (and universal) exclusivity between low frequency fidelity and fast compressor release times. Stated another way: the faster a compressor's release time, the more distorted the lower frequencies will be. "Okay, okay!", you say, "I know that! But why don't I have similar problems with my other compressors?" Simply put, the RNC's normal mode release times are shorter than many compressors (some of the fastest that we've seen). This means that the RNC will induce low frequency distortion more frequently than your other compressors. "Why didn't you make the RNC so it wouldn't distort my bass notes?" Because then we'd limit (no pun intended) your creative choices for other sound sources where a really fast release time would sound really gonzo...like on kick or snare drums. Try compressing a snare drum track with the RNC set for really fast attack and release times. You'll here drum resonances that you've never heard before that can be creatively used to add spice to your mixes!

"How do I avoid or reduce the low frequency distortion?" This one's easy: increase the release time until the distortion goes away. (Doing my best Groucho Marx impression: "Does it distort when you do that? Well don't do that!")

Man, I read that like four years ago and totally forgot it existed! Thanks for that Gilliland.