You mentioned the piano is only in the middle. So, let it be the star in that section and have the bass lay back. To do that I'd consider multiband compression, and compress just the low end of the bass in that part of the song. I'd use quite a bit so there's plenty of sustain that allows bringing down the bass level somewhat while still having it be clearly audible. Then I'd take off a lot of the top off the bass while the piano is playing, as the piano will have more harmonics that "imply" its lower frequencies.
 
Pan both the bass and piano to center as you experiment with these settings. Once you can differentiate them well in mono, go for stereo.

Another thought depending on the song style and the feel you want at this particular section: perhaps try removing the bass part through this section - just piano, vocals, & soft drums? This might give some nice contrast and interest to the song as a whole.

A few words for you, 'Complimentary EQ Techniques.' These are the 3 words that make up a phrase that you should learn.
 
Cj

Thanks et all for the advice. Just thought I would post that I have tried a lot of things mentioned here and had some good success. Pretty much a combination of all things of course, but I have essentially split the piano hands into separate tracks, applied a very small amount of side chaining to the bass piano parts from the bass line, used the multiband compressor on the bass, re-eq'd everything again, and panned until things felt right. I did re-do the bass for this section too, so it wasn't so busy, and sat more on the piano notes, plus mucked around with velocity's and attack times.
 
I think more than anything, it was the compression in the end that helped to get things under control, but I do take your point on EQ'ing Cj. I think some times you have to walk away and come back fresh, get your ears unblocked or something.
 
It's now sounding pretty good and the customer is stoked. What more could I ask for?
 
All the advice here was just great. Thanks so much.
 
Dave.

Sounds like you've got it whipped by now. One more thing you may consider is automating the EQ on the part that seems so offensive. Just write in some automation for the bands you need to tame for that section. If every one is happy at this point, follow the first rule of studio etiquette! "IF EVERYONE IS SMILING WHILE IT PLAYS-DON'T TOUCH IT!" LOL!!

Sidroe
Sounds like you've got it whipped by now. One more thing you may consider is automating the EQ on the part that seems so offensive. Just write in some automation for the bands you need to tame for that section. If every one is happy at this point, follow the first rule of studio etiquette! "IF EVERYONE IS SMILING WHILE IT PLAYS-DON'T TOUCH IT!" LOL!!

Ha!  I like that so much I might print it and frame it up on the wall Sidroe!! Yep. Shudda auto'd the EQ. Know that now lol!!  I used new tracks and did it that way. I think I'm an old 'desk' kinda guy. Hard to knock that out of myself, but I'm trying.....
 
Thanks for your help. Appreciate. 

I sympathize about the desk. I made myself put my mixers in storage when I bought my Studio-Captures. I 've been doing everything in the box for a little over a year. Just to see if I could adapt. It has taken some getting used to but I think I would find it hard to go back. Just like everything else, both ways have their pros and cons.
Anyway, I'm glad that you have resolved your problem and was very happy that I could help in any way! Go make some more music!