I would say that just about any decent bass will produce low end (within reason, of course...). Flat wound strings don't produce more low end, just less high end brightness. Each basses low end will sound a bit different. I like the p-bass low end "bounce" or round tone. My Carvin has a nice low end as well, but much more of a modern sound (ie: kinda scooped mids). If you run a really good bass into a small combo amp with a single 10" speaker, don't expect great lows.
I would say that for really deep low end, you will want part of your bass rig to emulate the low end section of a large PA. Here's what I mean. Of course, this is subject to whatever your budget constraints are.
1 - have at least one 18" cabinet that can handle big wattage. Maybe even a PA subwoofer type cabinet (what the heck, let's go for it).
2 - Buy an electronic cross over and a BBE sonic maximizer. Take a line out signal from your current bass head into the sonic maximizer. Run the low end process knob all the way up.
3 - Take the output of the BBE into the input of the crossover.
4 - Set the crossover to cross the lows at about 150 Hz.
5 - Take the low end output of the crossover into a big, honkin' power amp. Something that can produce 300W per side at 8 ohms. Put the amp in bridge mode, and then the output into the 18" subwoofer cabinet.
Voila! You will now have sick low end which you can adjust seperately from your regular sound, and your regular bass sound will still be coming out of your regular bass rig and cabinet.
:^)~