I've never used any of Ritter's string sets, but the Slick Rounds are a groundwound (like D'Addario Half Rounds). About any groundwound will get you close, but I prefer the Ken Smiths to the D'Addarios. GHS also makes Brite Flats - I've read here on Talkbass that GHS makes Ken Smith Slicks, they're just rebranded (and one has a tapered "B" string and one doesn't), and SIT makes Power Flats - all groundwounds. I want to grab a set of the SIT's to try with my next string order since they're a local company. Compression wounds are a similar flavor as well - Ken Smith Compressors, SIT Silencers, GHS Compression Wound, etc. - all will do a better Phil than rounds.
I'm currently using D'Addario Chrome flatwounds and then EQ some bass out and increase the treble a tad and then open up the tone control on my P bass and I can get a nice "woody" tone that way and still have some good low end.

