Hi Guys.
Thanks Dave! Just between you and me, there ought to be a law against having so much fun.

I'm not exaggerating when I say my soloist project is exceeding my expectations. It's been an interesting year to say the least.
Rich - Heya, long time no see and thank you. To try and answer your question, (congrats on the JamMan btw), have a seat and let me first establish my setup.
My looper and synth patches are 'tied together' via midi, so if I select patch #50 on the looper, the synth tracks the looper patch selection and changes to patch #50 also. So, I have my synth patch numbers matched up to my looper patch numbers, with the appropriate synth voice for the tune, to keep button pushes and knob spins to a minimum. Cool?
In this video I believe I first changed my synth patch, as 'Coppin a Feel' is patch #50 on the looper and the sax/synth patch I used is on patch #29, so I had to manually change the synth patch from #50 to #29. I can do this via the midi controls on my bass, but as it would take 21 pushes of a button, it was quicker to reach down and 'spin a knob'. As I said in the previous paragraph normally I'd have the patches matched up, but as this was an 'impulsive/improv' sax solo, I wasn't prepared in advance.
So, I went to patch #29 on the synth to play the sax patch/voice I wanted and then I believe I bumped the input level on the looper up some to bring the sax volume up some more.
Whew, it's so much easier to do then to explain. No prob though.
Realize most of the above applied to this video only.
I remember when I had my RC-20XL I'd have a problem that sounds similar to what you're describing.
What I'd do is record the first loop a lot hotter via volume control on my bass. The second one would be a little less 'hot' than the first, but hotter than the third loop etc, etc.

After doing it like this I kinda got a feel for volume settings and adjustments and it did help in keeping the first parts 'coming through the mix'. The hardest part was remembering to keep turning the volume down for each new looper part.
On my RC-50 looper I have three separate channels/phrases I can record on, each with an individual/independent volume control, as well as an overall master volume. If I'm 'prepared' I'll have the volumes set to do as in the previous paragragh, with the volumes matched with each other without having to make adjustment. I believe the JamMan stacks each part on top of the previous one(s). Major difference between the two loopers, if I'm correct.
However for me, I'm generally not that 'together' so I *do* find myself adjusting individual channel/phrase volumes to keep them balanced. This is most likely what you've see me doing when I'm reaching down and doing something or another.

I don't have any EQ control on the looper itself just as an FYI.
It also helps to play your parts so they happen at different times in your groove. In this video (and on some of the others I've done), you can see/hear

how I'm doing this with the first two parts as I put down the bass line and then the 'guitar' part. The third part is built around the first bass part and the second guitar part, but as you mentioned in your message about different ranges, you'll note it's one octave higher then the bass part/first loop.
And I think that answers your questions and as always I'm happy to try and help as best as I can. I'm only sorry I can't seem to keep it less wordy and easier to follow. Feel free to ask for clarification.
With (all) that, it's getting late so I'm off to sleepy time.
Nice to hear from you Rich. All my best.
Take care of yourself Dave. Be seeing ya 'round.
Best Regards,
Kimberly