I have an RSPdx and run it mainly with SDRuno, largely because the combination is the only one that gives access to the low-band hardware filters. SDRuno is a bit clunky but, interestingly, I find that, properly adjusted, its noise reduction algorithm is actually pretty good - better than many ham transceivers. I also have a West Mountain CLRdsp unit able to be allocated to any audio stream, including the RSPdx/PC. I'm not a gushing fan of that unit: it's OK and, with careful setting, helps a bit with classic radios in the presence of some types of noise. I'm not disgruntled with it by any means - just not fully gruntled, either. There are some great DSP algorithms around, including the well-known Apache labs variations but these boxes are a long way from those: as you'd expect, of course, given the baseband domain in which they operate and the lack of programmability. I suspect that a lot of the favourable impression one gets on first try is just from the low-pass and spectrum shaping effects. Depending on the type of noise you're dealing with, you may or may not find a particular external audio processor useful. In my case, I've not found that the addition of the CLRdsp box adds much worthwhile to the SDRuno implementation. Any way you could arrange a trial of your favoured external box?
73, Peter.