Same. I have the Interbass too, and I've never thought of putting it into the v803. I don't think it would really add more EQ options, and I don't see the point of stacking 2 layers of "light grit" - I also prefer using a dirt pedal. I should try though, just for the sake of "science".
You inspired me to try that as well for "science"!
On a related note, I have a Source Audio Zio Bass that I really like but when I got the V803, after a lot of experimenting. I felt I was getting pretty close to my preferred Zio tone with the 803 alone and so I stopped using it and was getting ready to sell it.
Over the last few days though, I decided to put the Zio back in front and it's been incredible.
The Zio has a switch that explicitly tries to cop the Pultec trick and while Quilter doesn't say this anywhere, I think the scoop inputs is doing something similar, just a little less pronounced.
I know my explanation is probably not technically correct, but My understanding is the Pultec trick is not just a mid scoop, but also a bass boost emanating from two eq "controls" which broadly overlap, but importantly are slightly different.
This is a long way of saying if the "out of phase" or different frequencies, different q or whatever is going on in the Pultec is nice, why not try two of those eq options stacked, also slightly "out of phase" to each other and see what happens? That's what I did and I dig it.
Interbass into 803 might be the biz, could be terrible or just a big nothingburger.
We need more "scientists" … and also more scientists.