If you want definition when mixing bass you shouldn't boost the highs but the mids. Mids can of course mean anything from 200Hz to 3kHz (or even wider depending on who you ask) but depnding on the song/genre I would try somewhere between 700Hz to 1.5kHz for definition. Remember to eq the bass IN THE CONTEXT OF THE MIX, not soloed. That's the mistake most people make when mixing/eq'ing bass: The settings where the bass sounds best soloed is practically never the settings that actually work in the mix.
If you still want new strings there's always pure nickel roundwounds. They're almost as warm/smooth sounding as flats but generally have a bit more upper mid presence. They don't have the THUMPY character of flats though, they behave very much like roundwounds. My favourites are the Pyramid Pure Nickels closely followed by the GHS Balanced Nickels. Both are great. The Pyramids have a long break in period before they sound great so if you want fast results the GHS Balanced Nickels are the best choice.
If you want flatwounds with more upper mid presence the TI Jazz Flats are great. In fact, they are my favourite strings EVER. They have very loose tension and may seem a bit lacking in the lows until you get used to them and realize that they're actually PERFECTLY balanced. I can't think of any other bass strings that sit better in the mix than the TI Jazz Flats. Still, they are so unique and utterly different from any other strings out there that they're not for everyone. It's very much love or hate with the TI Jazz Flats.
I've never found a "halfway" type of strings I liked. Bright flats, pressurewounds, groundwounds, half-rounds... whatever they are called they all sound horrible to me but lots of people like them so YMMV.
Thanks for the EQ tips and suggestion for TI Jazz Flats. I do mix in context, as well as solo, so I can hear what I am doing to the bass. I assume one advantage with going with a roundwound is that it is easier to dial back the mid/highs then to add them later in the mix. I assume you are not a fan of EB Colbalt strings?