Hey Gary -
As a blues player of more decades than I want to think about right now, I personally love the P for blues, though I don't own one. My main bass is a G&L L-1000 which I use mostly in single coil position, and that's pretty P-like. I've never used a Mark Bass, but looking over the manual it looks to me like you've got plenty of tone shaping options without having to add anything external. It could be that you are already over-eq'ing, not under eq'ing.
Here's what I'd do. All of this is VERY approximate since I've never used your rig, I'm just trying to show you how to think it out. Start with the 4 band eq all at 12:00, and the VLE and VPF off. Listen to your sound and see what you think you need. If you want more lows, see what just the VLE does for you first. That'll roll off high mids and up and get things to sounding rounder. Let's just say try going for something less than 12:00. Still want more lows - see if boosting the low eq a bit gets what you want. If you're having to boost low's more than say 3:00 to even start getting close, I'd set it at maybe 2:00 and then see what adding a little of the VPE does. That'll boost lows and highs and scoop mids. Scooping the mids makes more room for the lows. Again, not too much - under 12:00. Now if that's too bright, roll the highs back a bit. Want more lows still - try rolling the low mid eq back some and boosting low eq a bit more.
So what you're doing is starting with the controls where you know you're hearing just your bass (flat), then use the controls in combination and each one as little as possible to get what you want. Don't feel like you're doing something wrong if you end up getting what you want with just the VLE, VPE and low eq. So many players feel compelled to do something with every band of eq they have, when the real goal should be to leave as much of it as possible flat. Don't be afraid to use as much eq as you need, or use an external one if you need to, but also don't be afraid to use as little as you need.
Also understand that just because an eq can boost or cut 15dB does not mean it is intended to be used that much. Filter designs utilize headroom just like pre and power amp designs do. Any eq, no matter how good, gets weird once you go very much further than 4dB of boost or cut, and turns to paper if you approach 6dB, and if you listen you also began loosing things in frequencies outside of what you're eq'ing - for instance over-eq'ing low's will cause upper mids to begin getting strange and phasey - it's called phase distortion. So if you're having to use a lot of eq, don't just hyper-focuse on the frequencies you're boosting or cutting, pay close attention to what it's doing outside of that.
Another thing is, you might have too much bottom going on, not too little. Sometimes you need more mids and high mids than you think, because having those as CONTRAST makes the bottom end clearer and more defined. It could also be that what you are really wanting is sub-lows, and your combo just isn't capable of reproducing the frequencies you're wanting to hear/feel.
