| Hi JMG,
Sounds like there is some sort of connection problem - most probably the DBX since there's nothing from it straight into your amp...
The best way to run rack compressors is through the effects loop, as previously suggested, but if your dbx doesn't even work in front of the amp, best to get another compressor.
I like pedal compressors more than rack ones, mostly because you can tweak your compression without altering any tone from your amp - more versatile, more straight-to-the-point.
The best pedal I have used is the DOD Milk-Box - incredible. Smoothens everything out beautifully, or keeps your bass sound natural, without any squish - an exceptional compressor.
I used rack gear for a while, but realised that compression tweaking meant changes to the eq, changing output level, changing this, that, etc...Pedals let you compress everything before letting your amp have the final say in your tone.
Hope this helps! All the best,
JOHN. | 
